• Related: Pushing blobs doesn't send content-type moby/moby#46006 (comment)
  Credit to @makkes for finding the bug and identifying the root cause.

When httpBlobUpload is used as an io.Writer, each Write() call issues a PATCH request to the remote registry with Content-Type, Content-Range and Content-Length headers. However, if it is used as an io.ReaderFrom, each ReadFrom() call issues a PATCH request with none of those headers. While the Content-Range and Content-Length header values would be challenging to implement without introducing buffering, there's no good reason to also omit the Content-Type header.