Prevent crash if uploaded file contains # character

[matthias-ronge]

Image upload in metadata editor crashes, if the filename is i.e. A-MG_UTTA0#102_OSQ0#62317_12052021_1.tif.

The stack trace indicates that there is an URL encoding issue:

Goal: Either, forbid uploading files with disallowed characters (and show an error message), or handle them. IMHO it would also be okay to replace the # with a _ or that like, if it causes problems.