diff --git a/app/main/views/send.py b/app/main/views/send.py
index cf64b8692..43b43f197 100644
--- a/app/main/views/send.py
+++ b/app/main/views/send.py
@@ -187,7 +187,6 @@ def send_messages(service_id, template_id):
             ))
 
     column_headings = get_spreadsheet_column_headings_from_template(template)
-    allowed_spreadsheet_file_extensions = ','.join([f'.{ext}' for ext in Spreadsheet.ALLOWED_FILE_EXTENSIONS])
 
     return render_template(
         'views/send.html',
@@ -195,7 +194,7 @@ def send_messages(service_id, template_id):
         column_headings=list(ascii_uppercase[:len(column_headings)]),
         example=[column_headings, get_example_csv_rows(template)],
         form=form,
-        allowed_spreadsheet_file_extensions=allowed_spreadsheet_file_extensions
+        allowed_file_extensions=Spreadsheet.ALLOWED_FILE_EXTENSIONS
     )
 
 
@@ -711,7 +710,7 @@ def _check_messages(service_id, template_id, upload_id, preview_row, letters_as_
 def check_messages(service_id, template_id, upload_id, row_index=2):
 
     data = _check_messages(service_id, template_id, upload_id, row_index)
-    data['allowed_spreadsheet_file_extensions'] = ','.join([f'.{ext}' for ext in Spreadsheet.ALLOWED_FILE_EXTENSIONS])
+    data['allowed_file_extensions'] = Spreadsheet.ALLOWED_FILE_EXTENSIONS
 
     if (
         data['recipients'].too_many_rows
diff --git a/app/main/views/uploads.py b/app/main/views/uploads.py
index d65ce6ffe..b2609bde6 100644
--- a/app/main/views/uploads.py
+++ b/app/main/views/uploads.py
@@ -292,7 +292,6 @@ def uploaded_letter_preview(service_id, file_id):
     form = LetterUploadPostageForm(
         postage_zone=postal_address.postage
     )
-    allowed_spreadsheet_file_extensions = ','.join([f'.{ext}' for ext in Spreadsheet.ALLOWED_FILE_EXTENSIONS])
 
     template = get_template(
         template_dict,
@@ -314,7 +313,7 @@ def uploaded_letter_preview(service_id, file_id):
         message=error_message,
         error_code=error_shortcode,
         form=form,
-        allowed_spreadsheet_file_extensions=allowed_spreadsheet_file_extensions,
+        allowed_file_extensions=Spreadsheet.ALLOWED_FILE_EXTENSIONS,
         postal_address=postal_address,
         re_upload_form=re_upload_form
     )
@@ -379,7 +378,6 @@ def send_uploaded_letter(service_id, file_id):
 @user_has_permissions('send_messages')
 def upload_contact_list(service_id):
     form = CsvUploadForm()
-    allowed_spreadsheet_file_extensions = ','.join([f'.{ext}' for ext in Spreadsheet.ALLOWED_FILE_EXTENSIONS])
 
     if form.validate_on_submit():
         try:
@@ -416,7 +414,7 @@ def upload_contact_list(service_id):
     return render_template(
         'views/uploads/contact-list/upload.html',
         form=form,
-        allowed_spreadsheet_file_extensions=allowed_spreadsheet_file_extensions,
+        allowed_file_extensions=Spreadsheet.ALLOWED_FILE_EXTENSIONS,
     )
 
 
@@ -451,7 +449,6 @@ def check_contact_list(service_id, upload_id):
     )
 
     non_empty_column_headers = list(filter(None, recipients.column_headers))
-    allowed_spreadsheet_file_extensions = ','.join([f'.{ext}' for ext in Spreadsheet.ALLOWED_FILE_EXTENSIONS])
 
     if len(non_empty_column_headers) > 1 or not template_type or not recipients:
         return render_template(
@@ -460,7 +457,7 @@ def check_contact_list(service_id, upload_id):
             original_file_name=original_file_name,
             template_type=template_type,
             form=form,
-            allowed_spreadsheet_file_extensions=allowed_spreadsheet_file_extensions
+            allowed_file_extensions=Spreadsheet.ALLOWED_FILE_EXTENSIONS
         )
 
     if recipients.too_many_rows or not len(recipients):
@@ -469,7 +466,7 @@ def check_contact_list(service_id, upload_id):
             recipients=recipients,
             original_file_name=original_file_name,
             form=form,
-            allowed_spreadsheet_file_extensions=allowed_spreadsheet_file_extensions
+            allowed_file_extensions=Spreadsheet.ALLOWED_FILE_EXTENSIONS
         )
 
     row_errors = get_errors_for_csv(recipients, template_type)
@@ -480,7 +477,7 @@ def check_contact_list(service_id, upload_id):
             original_file_name=original_file_name,
             row_errors=row_errors,
             form=form,
-            allowed_spreadsheet_file_extensions=allowed_spreadsheet_file_extensions
+            allowed_file_extensions=Spreadsheet.ALLOWED_FILE_EXTENSIONS
         )
 
     if recipients.has_errors:
@@ -489,7 +486,7 @@ def check_contact_list(service_id, upload_id):
             recipients=recipients,
             original_file_name=original_file_name,
             form=form,
-            allowed_spreadsheet_file_extensions=allowed_spreadsheet_file_extensions
+            allowed_file_extensions=Spreadsheet.ALLOWED_FILE_EXTENSIONS
         )
 
     metadata_kwargs = {
diff --git a/app/templates/views/check/column-errors.html b/app/templates/views/check/column-errors.html
index 599ddc59c..f576ae305 100644
--- a/app/templates/views/check/column-errors.html
+++ b/app/templates/views/check/column-errors.html
@@ -137,7 +137,7 @@
       {% if not request.args.from_test %}
         {{ file_upload(
           form.file,
-          allowed_file_extensions=allowed_spreadsheet_file_extensions,
+          allowed_file_extensions=allowed_file_extensions,
           action=url_for('.send_messages', service_id=current_service.id, template_id=template.id),
           button_text='Upload your file again'
         ) }}
diff --git a/app/templates/views/check/row-errors.html b/app/templates/views/check/row-errors.html
index a971047ae..49f7dc7cb 100644
--- a/app/templates/views/check/row-errors.html
+++ b/app/templates/views/check/row-errors.html
@@ -43,7 +43,7 @@
     <div class="form-group">
       {{ file_upload(
         form.file,
-        allowed_file_extensions=allowed_spreadsheet_file_extensions,
+        allowed_file_extensions=allowed_file_extensions,
         action=url_for('.send_messages', service_id=current_service.id, template_id=template.id),
         button_text='Upload your file again'
       ) }}
diff --git a/app/templates/views/email-branding/manage-branding.html b/app/templates/views/email-branding/manage-branding.html
index 965ea3c81..f90f0bc47 100644
--- a/app/templates/views/email-branding/manage-branding.html
+++ b/app/templates/views/email-branding/manage-branding.html
@@ -24,7 +24,7 @@
       <p class="govuk-body">
         Logos should be PNG files, 108px high
       </p>
-      {{ file_upload(form.file, allowed_file_extensions='.png', button_text='{} logo'.format('Update' if email_branding else 'Upload')) }}
+      {{ file_upload(form.file, allowed_file_extensions=['png'], button_text='{} logo'.format('Update' if email_branding else 'Upload')) }}
       {% call form_wrapper() %}
         <div class="form-group">
           <div style='margin-top:15px;'>{{form.name}}</div>
diff --git a/app/templates/views/letter-branding/manage-letter-branding.html b/app/templates/views/letter-branding/manage-letter-branding.html
index 6903a740c..a7931ff3a 100644
--- a/app/templates/views/letter-branding/manage-letter-branding.html
+++ b/app/templates/views/letter-branding/manage-letter-branding.html
@@ -24,7 +24,7 @@
       <p class="govuk-body">
         Logos should be SVG files, cropped to artwork bounds and with all fonts outlined.
       </p>
-      {{ file_upload(file_upload_form.file, allowed_file_extensions='.svg', button_text='{} logo'.format('Update' if is_update else 'Upload')) }}
+      {{ file_upload(file_upload_form.file, allowed_file_extensions=['svg'], button_text='{} logo'.format('Update' if is_update else 'Upload')) }}
       {% call form_wrapper() %}
         <div class="form-group">
           {{ letter_branding_details_form.name(param_extensions={
diff --git a/app/templates/views/send.html b/app/templates/views/send.html
index 49081b035..6c8166727 100644
--- a/app/templates/views/send.html
+++ b/app/templates/views/send.html
@@ -19,7 +19,7 @@
   <div class="page-footer bottom-gutter">
     {{file_upload(
       form.file,
-      allowed_file_extensions=allowed_spreadsheet_file_extensions,
+      allowed_file_extensions=allowed_file_extensions,
       button_text='Choose a file'
     )}}
   </div>
diff --git a/app/templates/views/uploads/choose-file.html b/app/templates/views/uploads/choose-file.html
index dcf69b1bf..fd28a36b3 100644
--- a/app/templates/views/uploads/choose-file.html
+++ b/app/templates/views/uploads/choose-file.html
@@ -29,7 +29,7 @@
   <p class="govuk-body">
     {{ file_upload(
       form.file,
-      allowed_file_extensions='.pdf',
+      allowed_file_extensions=['pdf'],
       action=url_for('main.upload_letter', service_id=current_service.id),
       button_text='Upload your file again' if error else 'Choose file',
       show_errors=False
diff --git a/app/templates/views/uploads/contact-list/column-errors.html b/app/templates/views/uploads/contact-list/column-errors.html
index b69503e88..24c4c6a53 100644
--- a/app/templates/views/uploads/contact-list/column-errors.html
+++ b/app/templates/views/uploads/contact-list/column-errors.html
@@ -49,7 +49,7 @@
     <div class="form-group">
       {{ file_upload(
         form.file,
-        allowed_file_extensions=allowed_spreadsheet_file_extensions,
+        allowed_file_extensions=allowed_file_extensions,
         action=url_for('.upload_contact_list', service_id=current_service.id),
         button_text='Upload your file again'
       ) }}
diff --git a/app/templates/views/uploads/contact-list/row-errors.html b/app/templates/views/uploads/contact-list/row-errors.html
index 0e9bd6be6..ba3fd1d14 100644
--- a/app/templates/views/uploads/contact-list/row-errors.html
+++ b/app/templates/views/uploads/contact-list/row-errors.html
@@ -44,7 +44,7 @@
     <div class="form-group">
       {{ file_upload(
         form.file,
-        allowed_file_extensions=allowed_spreadsheet_file_extensions,
+        allowed_file_extensions=allowed_file_extensions,
         action=url_for('.upload_contact_list', service_id=current_service.id),
         button_text='Upload your file again'
       ) }}
diff --git a/app/templates/views/uploads/contact-list/too-many-columns.html b/app/templates/views/uploads/contact-list/too-many-columns.html
index 313e14d47..abc28d08d 100644
--- a/app/templates/views/uploads/contact-list/too-many-columns.html
+++ b/app/templates/views/uploads/contact-list/too-many-columns.html
@@ -58,7 +58,7 @@
     <div class="form-group">
       {{ file_upload(
         form.file,
-        allowed_file_extensions=allowed_spreadsheet_file_extensions,
+        allowed_file_extensions=allowed_file_extensions,
         action=url_for('.upload_contact_list', service_id=current_service.id),
         button_text='Upload your file again'
       ) }}
diff --git a/app/templates/views/uploads/contact-list/upload.html b/app/templates/views/uploads/contact-list/upload.html
index a4c487e7e..93f7a8140 100644
--- a/app/templates/views/uploads/contact-list/upload.html
+++ b/app/templates/views/uploads/contact-list/upload.html
@@ -35,7 +35,7 @@
 <div class="bottom-gutter">
   {{ file_upload(
     form.file,
-    allowed_file_extensions=allowed_spreadsheet_file_extensions,
+    allowed_file_extensions=allowed_file_extensions,
     button_text='Upload your file again' if error else 'Choose file',
     show_errors=False
   )}}
diff --git a/app/templates/views/uploads/preview.html b/app/templates/views/uploads/preview.html
index 10588b153..1c74f6635 100644
--- a/app/templates/views/uploads/preview.html
+++ b/app/templates/views/uploads/preview.html
@@ -19,7 +19,7 @@
         <div class="form-group">
           {{ file_upload(
             re_upload_form.file,
-            allowed_file_extensions='.pdf',
+            allowed_file_extensions=['pdf'],
             action=url_for('main.upload_letter', service_id=current_service.id),
             button_text='Upload your file again'
           ) }}
diff --git a/app/utils.py b/app/utils.py
index 19ad0668b..a983fe97a 100644
--- a/app/utils.py
+++ b/app/utils.py
@@ -291,7 +291,7 @@ def id_safe(string):
 
 class Spreadsheet():
 
-    ALLOWED_FILE_EXTENSIONS = {'csv', 'xlsx', 'xls', 'ods', 'xlsm', 'tsv'}
+    ALLOWED_FILE_EXTENSIONS = ('csv', 'xlsx', 'xls', 'ods', 'xlsm', 'tsv')
 
     def __init__(self, csv_data=None, rows=None, filename=''):
 
diff --git a/tests/app/main/views/test_send.py b/tests/app/main/views/test_send.py
index 4ae3d3754..e56baeaef 100644
--- a/tests/app/main/views/test_send.py
+++ b/tests/app/main/views/test_send.py
@@ -309,8 +309,7 @@ def test_example_spreadsheet(
         '1 phone number name date'
     )
     assert page.select_one('input[type=file]').has_attr('accept')
-    assert set(page.select_one('input[type=file]')['accept'].split(",")) ==\
-        {'.csv', '.xlsx', '.xls', '.ods', '.xlsm', '.tsv'}
+    assert page.select_one('input[type=file]')['accept'] == '.csv,.xlsx,.xls,.ods,.xlsm,.tsv'
 
 
 def test_example_spreadsheet_for_letters(
@@ -511,8 +510,7 @@ def test_upload_csv_file_with_errors_shows_check_page_with_errors(
 
     page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
     assert page.select_one('input[type=file]').has_attr('accept')
-    assert set(page.select_one('input[type=file]')['accept'].split(",")) ==\
-        {'.csv', '.xlsx', '.xls', '.ods', '.xlsm', '.tsv'}
+    assert page.select_one('input[type=file]')['accept'] == '.csv,.xlsx,.xls,.ods,.xlsm,.tsv'
 
     content = response.get_data(as_text=True)
     assert 'There’s a problem with example.csv' in content
@@ -906,8 +904,7 @@ def test_upload_csv_file_with_missing_columns_shows_error(
         assert 'file_uploads' not in session
 
     assert page.select_one('input[type=file]').has_attr('accept')
-    assert set(page.select_one('input[type=file]')['accept'].split(",")) == \
-        {'.csv', '.xlsx', '.xls', '.ods', '.xlsm', '.tsv'}
+    assert page.select_one('input[type=file]')['accept'] == '.csv,.xlsx,.xls,.ods,.xlsm,.tsv'
     assert normalize_spaces(page.select('.banner-dangerous')[0].text) == expected_error
 
 
diff --git a/tests/app/main/views/uploads/test_upload_contact_list.py b/tests/app/main/views/uploads/test_upload_contact_list.py
index 9915d0696..5896d5da6 100644
--- a/tests/app/main/views/uploads/test_upload_contact_list.py
+++ b/tests/app/main/views/uploads/test_upload_contact_list.py
@@ -18,8 +18,7 @@ def test_upload_contact_list_page(client_request):
     assert 'action' not in page.select_one('form')
     assert page.select_one('form input')['name'] == 'file'
     assert page.select_one('form input')['type'] == 'file'
-    assert page.select_one('form input').has_attr('accept')
-    assert set(page.select_one('form input')['accept'].split(',')) == {'.csv', '.xlsx', '.xls', '.ods', '.xlsm', '.tsv'}
+    assert page.select_one('form input')['accept'] == '.csv,.xlsx,.xls,.ods,.xlsm,.tsv'
 
     assert normalize_spaces(page.select('.spreadsheet')[0].text) == (
         'Example A '
@@ -236,8 +235,7 @@ def test_upload_csv_file_shows_error_banner(
         service_id=SERVICE_ONE_ID,
     )
     assert page.select_one('form input')['type'] == 'file'
-    assert page.select_one('form input').has_attr('accept')
-    assert set(page.select_one('form input')['accept'].split(',')) == {'.csv', '.xlsx', '.xls', '.ods', '.xlsm', '.tsv'}
+    assert page.select_one('form input')['accept'] == '.csv,.xlsx,.xls,.ods,.xlsm,.tsv'
 
     assert normalize_spaces(page.select_one('thead').text) == expected_thead
     assert normalize_spaces(page.select_one('tbody').text) == expected_tbody