Fix file upload

[wemeetagain]

In order to do file upload more robustly, we want to stream files to be uploaded. We're using multipart-vfile-tree to ease the burden of managing file streams and multipart boundaries, especially burdensome in the case of uploading whole directory trees.

It looks like dexador, the http client we're using, does not currently support streaming uploads [1] [2] [3]