Allow uploading multiple files from your computer at once.
The maximum limit is 100 files at once. There is still a 50MB size limit
that applies to the whole upload. This limit is at the Nginx level.
The upload widget no longer shows a thumbnail preview of the uploaded
file. This is because there isn't room for it in a multi-file upload,
and because the next page will show a preview anyway after the files are
uploaded.
Direct file uploads are processed synchronously, so they may be slow.
API change: the `POST /uploads` endpoint now expects the param to be
`upload[files][]`, not `upload[file]`.
202dfe5d87