Both the admin and mform libraries give you the ability to constrain the types of file that will be accepted for upload.

The form will validate this to ensure the user uploads the right type of file.

However, the messages returned aren't as helpful as they could be, as they only tell you that the file you have uploaded is the wrong type, but don't tell you what the expected types are.

The mform filemanager already displays this information below the filemanager, though the admin version doesn't, which should probably be harmonised. Repeating this information in the form feedback pop-up that displays the error would help to draw the users attention to what they can do to proceed.