The FileTransferManager in MatriX is using IBB which supports only 1:1 transfers. There is currently no standard for transferring a file to multiple users at once in XMPP. Multicast file transfer is also a very complex topic.
I would suggest to use OOB (Out of Band Data) for file transfer to multiple users. With OOB you can upload the file to a HTTP, Webdav, Dropbox or something similar, and send the download link for the file to multiple users.