Check if all required fields are filled before file upload

I have a webform with file fields. I found a little problem today:

If someone fills that webform (including several big files / images) and forgets to fill one required field, then the files / images are uploaded first (takes several minutes) before the error message shows up that a required field is missing.
Then you have to upload everything again…

Is it somehow possible to check before the upload of files if all required fields are filled?