MDL-50794 workshop: Improve the file type restricting implementation
This is basically a clean up and what I think improved version of the
original Mahmoud's patch.
The actual checking for allowed file extensions has been re-implemented
and is now covered by unit tests. The list of allowed extensions is now
also assed to the filemanager element's accepted_types option to prevent
picking other files (we still need the in-place validation though). The
form validation is simplified a bit. The custom validation of file size
introduced in the previous patch has been removed as not related to this
issue (also I believe it should not be done at this level).