Outbound _matrix/federation/v1/media/download requests choke on lowercase HTTP headers #17544

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