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File transfer sure is hard. Why is it so hard?
By far my preferred system for 1-1 file transfer is the magic wormhole protocol with the rymdport cross-platform GUI frontend.
For 1-to-many transfer, I usually prefer Bittorrent.
For many-to-many, unfortunately cloud storage a la Google Drive / Dropbox etc appears to be the only real player in town.
If it's iOS to iOS and just 1-to-1 or 1-to-many, Airdrop works well. Many-to-many quickly becomes unmanageable.
For cross-platform iOS ↔ Android transfers, I usually reach for snapdrop. However, for transfers of more than 1gb or so, it starts to get sketchy with phone lock timeouts and background task kills frequently resulting in failed transfers.
I'm not aware of any reliable and broadly available tool that exists in this space. A magic wormhole client for mobile would be amazing. Do you know of one?