Allow uploading multiple files or an entire directory (tree)

[marcelgateau]

It would be nice to be able to upload multiple files at once.
As of now the frontend only allows adding a single file, both by drag-and-dropping and by selecting files manually. When trying to drag and drop a directory, the frontend raises an AbortError: The operation was aborted.

[tarneaux]

I guess the right way to do this is just like most other file sharing services: when you upload multiple files, they are placed into a zip file and then encrypted. When downloading you get the zip archive and decompress it yourself.
A nice bonus feature would be to allow using other formats: but this would mean storing the same file multiple times on the server side, which seems suboptimal and overcomplicated.