consider proxying/302ing large remote media files

[matrixbot]

This issue has been migrated from #3918.

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if Bob uploads a huge file to his HS, and Alice tries to fetch it, but it's too large for her homeserver, she'll just get an error ({errcode:M_TOO_LARGE,error:Requested file is too large > 104857600 bytes}). Could Alice's HS instead just proxy the request and stream the file from Bob's HS?

keywords: media, federation, limit, max_upload_size