Look into forms-form file error handling

This is motivated from an incident where a user uploaded an invalid file with a file name ".txt". The user was able to upload the file and get as far as the Reliability queue with the Vault processing - file check where it was flagged as invalid.

See https://gcdigital.slack.com/archives/C0BFB731K7Z

To main items for this ticket:

1. Double check same file validation logic is on infra and app
2. Double check the forms-form UI and see if there is a warning for invalid files? If not, add one.

Start by tracing the invalid-file validation in the forms-form UI, then compare the same logic across the infra and app layers. Review the incident linked in the issue for the .txt filename case. Done means validation is consistent and the UI warns users before an invalid file reaches processing.

Written by the indexing model from the issue text.