Filenames that cause problems on other platforms should not be allowed

[wonder-sk]

Seen in the wild: a user has attached a picture (on Android) that had a questionmark ? in the filename - which is fine on linux-based systems, but not on windows. We should check for any well-known characters that may be causing difficulties on other systems, and rename files in the project to avoid this kind of issues...

https://stackoverflow.com/questions/1976007/what-characters-are-forbidden-in-windows-and-linux-directory-names