Despite having no form.errors, the FileRequired() validator fails during validation with Flask - wtforms.

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There could be several reasons for the validator to fail even if no form errors are raised. Some possible reasons include:

1. The HTML form code might be missing the "enctype='multipart/form-data'" attribute. This attribute is required when working with file uploads.

2. The name attribute of the file input field might be incorrect. It should match the name of the field in the form class.

3. The request.files dictionary might not contain the file that was uploaded. This can happen if the file was not included in the POST request or if the wrong field name was used.

4. The file might be too large, and the server configuration might be set to reject files over a certain size. In this case, Flask might return a 413 error instead of raising a form error.

To debug the issue, you can try printing the contents of the request.files dictionary and checking if the file is present. You can also check the HTTP status code of the response to see if a different error is being returned.