If you submit the form while a file upload is in progress, then the form submits, but without the half-uploaded file. This is particularly dangerous if the anti-virus scanning is a bit slow, because it looks like the progress bar is complete, but there are several seconds delay. If you know what is going on, then you know to wait for the progress bar to be replaced by an icon, but if you are a novice user, it is easy to make a mistake.

Expected result: If you click the submit button while a file upload is in progress, then there is some sort of warning. For example, we could have an alert like:

A file upload is in progress. If you submit the form now, the file will be lost

(Cancel upload) (Wait for upload to complete)