Hey! Just tried to see this link to know what to pass by parameter on filepicker.
http://cvs.moodle.org/moodle/lib/filestorage/file_types.mm
Found this link on this other one
https://docs.moodle.org/dev/Using_the_File_API_in_Moodle_forms#filepicker
Ctrl+F for 'accepted_types'