This happens with some things in QuickBase. After you upload the file you need to interact with the form in some way, pick something from a dropdown or enter some text, and you will see the form rule suddenly trigger. This can be a little annoying. It's similar to how when you enter a date into a field nothing will trigger until you click outside the field.
Are there any other things on the form? You might be able to get away with placing the File Attachment on the form in a better spot so that the user uploads the file and has to take other actions after that before getting to your date field. Maybe they could categorize the upload in a dropdown, something. I know this isn't the best solution but it might work.