Stores a file and returns the id to reference it with in a later call. The file is the raw request body and must not exceed 52428800 bytes (50 MB). Its 'Content-Length' header is optional, but when it is set it must be the exact byte size of the body: a length smaller than the body stores a truncated file. Uploading requires the same permission as saving a question, which is the only call an upload can be referenced by. An upload no later call has referenced is eventually deleted, so it should be referenced promptly.