For various reasons, it is useful to be able to remove pieces of media from the local content repo. Ideally we'd have an API to do this, which also removes any associated thumbnails.

We have the quarantine_media API, but (a) that applies to rooms rather than individual bits of media (#5956), and (b) it doesn't actually delete the media.

It's also somewhat separate to the ability of users to remove their own media (see https://github.com/matrix-org/matrix-doc/issues/790), which again probably wouldn't actually delete the media files (to allow for moderation), and would send a deletion request over federation.

It's closely related to matrix-org/matrix-spec-proposals#3479, though that is more about finding the media.