Prevent creating part files from the UI

:construction: 
Fixes #7496 

This PR prevents users to create part files from the web UI.

Since we changed an important utility method (isValidFileName), a few things need to be retested:
- [ ] Test that WebDAV upload still works
- [ ] Test that sync client upload still works with small files (part files)
- [ ] Test that sync client upload still works with small files (part files + chunking)
- [ ] Test with external storage using a backend ownCloud (sync upload must still work)
- [ ] Improve error messages ? [minor]