Add non-multipart replacement for the /upload endpoint
Currently, backend's /upload
endpoint expects uploaded data with multipart/form-data
encoding. See the implementation here. This approach has some limitations (more complex encoding, temporary copies required by Java's multipart-implementation) and makes later replacement with S3 more difficult.
As an intermediate step, a new non-multipart-based /uploads
endpoint should be introduced, accepting a single uploaded file instead of multiple ones.