`TestMediaFilenames` incorrectly tests Content-Disposition

[Half-Shot]

Content-Disposition is not a spec defined behaviour (it's not mentioned in https://spec.matrix.org/v1.7/client-server-api/ other than for the filename, and complement should not be prescribing that servers act a certain way without a spec to cover it.

It's up to homeservers to decide what media types may be presented inline or as attachments (and something we're trying to fix in matrix-org/synapse#15988 to fix a unsatisfactory situation).

[Half-Shot]

This appears to have originally come from sytest, which also is testing this behaviour wrongly. #639.

Ultimately I'd be in favour of at most checking Content-Disposition is a valid value, but realistically we can't expect developers to pass these tests until the spec defines the requirements.