The file could not be loaded. Reason: The file extension does not match its underlying type.

RE: The file could not be loaded. Reason: The file extension does not match its underlying type.

Mike Schmitt — Tue, 14 Oct 2025 17:34:51 GMT

There's a longstanding and known bug where, for instance, if a user tries to upload a document that's actually empty (which Appian disallows), that then even after they clear that upload and try uploading a valid file, the error message still (incorrectly) persists. It sounds like, if what Eleeeeeeeeena is saying here is true, a similar (incorrect) behavior might be persisting on the "document type mismatch" error, which I haven't confirmed (or tested) for myself, but would not surprise me whatsoever.

RE: The file could not be loaded. Reason: The file extension does not match its underlying type.

Shubham Aware — Tue, 14 Oct 2025 09:28:32 GMT

Once you upload the correct file, the message should disappear. Make sure you are uploading the document to the same variable that is used in both the value and saveInto parameters.

RE: The file could not be loaded. Reason: The file extension does not match its underlying type.

Eleeeena1204 — Tue, 14 Oct 2025 08:27:12 GMT

Yes, that's correct, Appian shouldn't allow it. What happens is that the error message doesn't go away even when uploading a correct file. This disables the form and forces the user to refresh the entire page and start over. It's fine that the message may appear, but it shouldn't persist when uploading another correct file.

RE: The file could not be loaded. Reason: The file extension does not match its underlying type.

Shubham Aware — Mon, 13 Oct 2025 17:20:20 GMT

Changing a file extension manually does not alter the actual file content, so Appian blocks uploads when the extension and content don’t match. You cannot upload such files in Appian.

RE: The file could not be loaded. Reason: The file extension does not match its underlying type.

Eleeeena1204 — Mon, 13 Oct 2025 15:16:18 GMT

From my local desktop, what I do is create a new Excel file, which creates an empty document with an .xlsx extension (for example: test.xlsx):

If I attach it like this, everything is fine.
If I edit the name from the desktop using Windows Explorer and change the .xlsx to .pdf (or any other extension, I can also use .png; it's just to use the same file by manually changing its extension), the new name would be test.pdf, but it's the same file. So when I upload this same file with its new extension, I get that error message that I don't want, and I have to refresh the page and start over.
If I create a new file test2.pdf or any other extension and attach it, the message about my validation appears, not the one in the photo I showed in my question, which is also correct.

RE: The file could not be loaded. Reason: The file extension does not match its underlying type.

Shubham Aware — Mon, 13 Oct 2025 14:04:20 GMT

How you are changing file extension?

RE: The file could not be loaded. Reason: The file extension does not match its underlying type.

Eleeeena1204 — Mon, 13 Oct 2025 13:59:02 GMT

Hi! Thanks for your quick response, but I don't think that's the entire problem. If I upload a file that was originally a .pdf, it doesn't show this error, only my validation message. The problem lies in manually changing the file extension. If it was originally an .xlsx and I changed it to .pdf, it seems to become "corrupted" and the metadata doesn't match. It may have something to do with the administration, but I don't have access to it. However, .pdf files are accepted because they are attached elsewhere in the application. Thanks again.

RE: The file could not be loaded. Reason: The file extension does not match its underlying type.

Shubham Aware — Mon, 13 Oct 2025 13:04:17 GMT

This message appears because the PDF file type is blocked in the Admin Console under 'File Upload' using file extension blocking settings. The error will remain visible until the user manually refreshes or interacts with the interface; it does not clear automatically. One solution is to allow the file type at the admin level and then apply your own field-level validation in the interface(Make sure to create generic file upload to have consistency)