I have a form with an upload field that allows users to select multiple files. However, I need to be able to allow the user to select file 1 from folder 1, then go and select file 2 from folder 2, and so on.
Currently, when the user selects file 1 from folder 1 then hits "Open", the selection window closes (leaving the user on my form). Then if the user goes and select file 2 from folder 2 and hits the "Open" button, file 1 is removed, leaving only file 2.
Basically, the user is unable to select multiple files unless they're all in the same location. Is there a way to make file 1 stay selected after file 2 is chosen?
No, you can't. This is a behaviour defined by the operating systems and may vary between them. You can't control these things precisly and you will always fear what happen.
If the amount of folders people have to choose is quite small you could offer multiple upload fields.