Create Event Dialog: Forgetful recording elements section

[marcusm7]

The source dialog (files to upload for presenter/ presentation/ slides) forgets its settings when the user hits previous and next (or the other way around).

Steps:

• create new Event
• Select source: upload
• Choose videos to upload:
• hit previous-button, hit next button
• observe the missing videos:

[Arnei]

From what I can tell, this is because it is not possible to assign a file to a file input element, so we instead would have to style and display a different element (e.g. label) with the filename.

Before I now go and try to style a different component to look exactly like a file input element, I would like a second opinion on if my assessment is actually correct, and if there might be a better solution (possibly incooporating our use of Formik).

[Arnei]

Fixed by #1529