Currently I haven't licensed the backend as free software; the primary reason for this is that I reuse quite a lot of components from a different project of mine (https://gemin.ee) and I haven't decided yet on whether I want to release those components or remove them as dependencies. Since I too am familiar with useful stuff suddenly disappearing I'll give you my word that I will release the source if I have to shut down. If I decide to share the source I'll post a link to it on the site itself, so you won't have to look for it ;).
On another note, I'd encourage you to write a streaming download service. Compared to file uploading - storing - downloading I found it fun to do but also quite non-trivial to accomplish. I am planning to have another go at it myself, but with a different language (Go, Pony or Elixir). When run alongside of each other with the same UI/frontend/api it would make a nice benchmarking mechanism :).