Uploader: filter file_types glob

[tellyworth]

This makes it possible for a plugin to filter the types of files that are selectable by the user from the Flash uploader.

The value should be a string of semicolon-separated wildcards, like '*.jpg;*.mp3;*.txt'

The default is '*.*', same as in 2.5.0.

[ryan]

(In [7611]) upload_file_glob filter from tellyworth. fixes #6628 for trunk

[ryan]

(In [7612]) upload_file_glob filter from tellyworth. fixes #6628 for 2.5