Do not show postage stamp for letter preview

app/main/forms.py

@@ -775,7 +775,9 @@ class LetterUploadPostageForm(StripWhitespaceForm):
         default='second',
         validators=[DataRequired()]
     )
-    file_id = HiddenField()
+    file_id = HiddenField(
+        validators=[DataRequired()]
+    )
 
 
 class ForgotPasswordForm(StripWhitespaceForm):

app/main/views/uploads.py

@@ -153,6 +153,10 @@ def uploaded_letter_preview(service_id, file_id):
 
     error = get_letter_validation_error(error_message, invalid_pages, page_count)
     template_dict = service_api_client.get_precompiled_template(service_id)
+    # Override pre compiled letter template postage to none as it has not yet been picked even though
+    # the pre compiled letter template has its postage set as second class as the DB currently requires
+    # a non null value of postage for letter templates
+    template_dict['postage'] = None
 
     form = LetterUploadPostageForm()