Remove set-to-string conversion from upload code

Also changes the allowed_spreadsheet_file_extensions
variable to allowed_file_extensions for
consistency.

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