Add option to overwrite files

[LeonVreling]

Currently, the names of the uploaded files are hardcoded. This is something nice, and should be kept if possible. This gives a weird situation, however, when you want to overwrite a previously uploaded file. The new file you want to save currently is not being saved, as the name already exists in the file.

A good solution would be a check to see if a file already exist with the same name, and then give a warning if the admin user really would like to overwrite the file. If this is indeed the case, the file is overwritten / the old one removed and the new one saved.