Make function SendOOB in vendor/github.com/matterbridge/go-xmpp/xmpp.go to send meesage with actual body according to https://github.com/42wim/matterbridge/issues/1381#issuecomment-774032477.
Remove the appendage ":" + fileInfo.URL in bridge/xmpp/xmpp.go. This will not modify the message so that the link to the file is not sent in the text message. Instead the media file is available as inline display in modern XMPP clients.
Moreover, send the media attachement first, then the message.