Member since Feb 2003 ·
4447 posts · Location: Germany
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I would go with Out of Band data. This is also the pattern which most group chat software like HipChat, Slack, WhatsApp and others are using.
1) Client upload the file to a storage location (Webserver, Azure Blob storage, Amazon S3, Dropbox etc....)
2) Client shared the link to the file over OOB with a group (1=>n)
Alex
PS: SI is not a file transfer method, it defines Stream Initiation and negotiation which should be used with any file transfer protocol.