Fix content length on federation `/thumbnail` responses

[H-Shay]

/_matrix/federation/v1/media/thumbnail/ responses were failing due to the endpoint returning the wrong content-length. This was due to the content-length being calculated off of the length of the original media file, rather than the thumbnail file.

Fixes #17518.

Complement test at https://github.com/matrix-org/complement/pull/732

[anoadragon453]

(Waiting for approval on https://github.com/matrix-org/complement/pull/732 before merging.)

[H-Shay]

@anoadragon453 https://github.com/matrix-org/complement/pull/732 is approved and merged - can we merge this as well? (sytest failure is a presence test, pretty sure it's not related).

[anoadragon453]

Sorry for the delay here - I've been away. This looks good to merge.