MDL-34290 repository_equella: do not download files when not needed
MDL-34290 repository_equella: do not download files when not needed

repository_equella::get_file_by_reference respects request timeouts, downloads only images (for thumbnail generation),
does not use cache_file class (content is cached in moodle filepool if needed);
also repository_equella has counter of unsuccessfull connect attempts and do not perform any more if 3 failed
(within one request)

dilawar/moodle