I asked this on Commons:Help desk but my question seems to be going unanswered, so it's probably not possible or a not very well known undocumented feature.
I know how to find all files from a user (Special:ListFiles/Example) and how to find all images with the word "squirrel" (squirrel). But how do I find all the files from User:Example with the word "squirrel"?
I know I could add all the files from User: Example to some category and use incategory, but that would just be a workaround.
This does not sound like something is wrong in the code base (a so-called "software bug"), but instead like a support request (how to change settings, questions how to do something, etc.).
As Wikimedia Phabricator/Maniphest is for bug reports and enhancement requests, generally please ask on the MediaWiki Support Desk or ask on a dedicated project/extension talk page if existing.
In this specific case this looks like a duplicate of T188125: Make it possible to search by page author /contributor/ uploader.