Investigate whether we can stream zip files

[zerebubuth]

Context is this comment from @rmarianski.

Possible issues: we sometimes need to get 2 files out of the zip - is there a callback per file that we can use? Also, (IIRC - this might be bad information) zip files have the directory block at the end of the file - does this mean we'd need to buffer it all in RAM? If so, how large is the largest file we're likely to download?