Opens a resumable upload session to send a picture or video of a purchase item straight to the storage, so an upload cut by the connection goes on from the bytes already stored instead of starting again. PUT the file to url in chunks with Content-Range (a status query Content-Range: bytes */<size> answers how many bytes are stored), then confirm the upload with path so it can be referenced from customerImageNames when the purchase is created. The session only accepts exactly size bytes and answers CORS requests for the origin of this request.