If file transfer is an issue with Briar, maybe #Jabber/#XMPP is better suited for our use case?
By default, modern clients use XEP-0363, i.e. file is uploaded to your server (typically encrypted) and download link plus decryption key is sent to the receiver.
However, files can also be transmitted directly using XEP-0234. This works only, if both parties are online at the same time. Clients can fall back to this method, e.g. if files are too large for the server.
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