Fix unsafe hotserving behaviour for non-multimedia uploads. (#15680)

* Fix unsafe hotserving behaviour for non-multimedia uploads.

* invert disposition assert

* test_media_storage.py: run lint

* test_base.py: /inline/attachment/s

* Only return attachment for disposition type, update tests

* Update synapse/media/_base.py

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

* Update changelog.d/15680.bugfix

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

* add attribution

* Update changelog.

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Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>