Search prototype currently accomplishes this by performing multiple negated filetype searches, for all non-selected filetypes (e.g. -filetype:text -filetype:office -filetype:audio ...),
but we really should use the recently introduced filetype:bitmap|audio syntax to search in multiple filetypes at once (see https://gerrit.wikimedia.org/r/c/mediawiki/extensions/WikibaseMediaInfo/+/441040/53/resources/search/SearchWidget.js#52)
Acceptance criteria:
This can be tested on https://federated-commons.wmflabs.org/wiki/Main_Page, in the top-right search widget.
- Selecting multiple file types and submitting the form takes you to Special:Search where the search term includes something in the form of filetype:<selected type>|<other selected type> instead of -filetype:<not selected type> -filetype:<other not selected type>