Server with the correct content-length.  Sending a file with 511 or less
  bytes, content-length 512 is used.  Sending a file with 513 - 1023 bytes,
  content-length 1024 is used.  Files with a length of a multiple of 512 Bytes
  show the correct content-length. Only these files work for upload.
  http://curl.haxx.se/bug/view.cgi?id=2057858