@roelvanlierop
I am *really* new to C5, just looking whether I'll use it or not, however your question is probably jQuery related, rather than C5 related - a bit of experience with this other side..
C5 is jQuery based, so the forms will be processed through that. I think if you look at the jQuery refresh (and error) options you may get your answer to clear fields, if your form DOES process with no errors. Your form should redirect to a processing page or script, then have a header (or similar) value to go back to the form, which reloads it and gives you a nice clean form.

Try placing a second upload / field entry AFTER the successful process on the form. If the second process doesn't complete it means the form is not completing.

Also - really basic thing is to check the code and make sure no silly MS word or other "rubbish" in the code.

Hope this helps a bit.

Actually - I have noticed this whole site does it - add something to your profile and nothing happens . . . . . except an alert at the top of the screen saying successful!! The screen doesn't even scroll to the top.
Perhaps this is just not your form, but the whole app. Perhaps a focus on submit as a minimum??