type http2writePushPromise struct {
	streamID uint32   // pusher stream
	method   string   // for :method
	url      *url.URL // for :scheme, :authority, :path
	h        Header

	// Creates an ID for a pushed stream. This runs on serveG just before
	// the frame is written. The returned ID is copied to promisedID.
	allocatePromisedID func() (uint32, error)
	promisedID         uint32
}

  <mime-type type="audio/opus">
    <_comment>Ogg Opus Codec Compressed WAV File</_comment>
    <magic priority="60">
      <!-- For a single stream file -->
      <match value="OggS\000.......................OpusHead" type="string"
             mask="0xFFFFFFFF00000000000000000000000000000000000000000000000000FFFFFFFFFFFFFFFF"
             offset="0"/>
    </magic>
    <glob pattern="*.opus"/>