Outbound `_matrix/federation/v1/media/download` requests choke on lowercase HTTP headers

[Xiretza]

Description

As reported here, outbound requests for the new authenticated media federation endpoints fail if the remote server responds with multipart headers that don't match the casing expected by synapse: #17365 (review)

HTTP and multipart headers are case-insensitive and should be handled as such.

Steps to reproduce

• Have a homeserver implementation that supports authenticated media, and responds with e.g. lowercase headers
• Try to look at media originating from this homeserver using a synapse account
• Media does not load

Homeserver

matrix.org

Synapse Version

1.113.0rc1 (b=matrix-org-hotfixes,46c66aa8e7)

Installation Method

I don't know

Database

I don't know

Workers

I don't know

Platform

I don't know

Configuration

I don't know

Relevant log output

Check with matrix.org admins for failed media requests to `blep.space`

Anything else that would be useful to know?

No response