nsIUploadChannel in Firefox 3.0

 Sets a stream to be uploaded by this channel.

 Most implementations of this interface require that the stream:
   (1) implement threadsafe addRef and release
   (2) implement nsIInputStream::readSegments
   (3) implement nsISeekableStream::seek

 History here is that we need to support both streams that already have
 headers (e.g., Content-Type and Content-Length) information prepended to
 the stream (by plugins) as well as clients (composer, uploading
 application) that want to upload data streams without any knowledge of
 protocol specifications.  For this reason, we have a special meaning
 for the aContentType parameter (see below).
 
 @param aStream
        The stream to be uploaded by this channel.
 @param aContentType
        If aContentType is empty, the protocol will assume that no
        content headers are to be added to the uploaded stream and that
        any required headers are already encoded in the stream.  In the
        case of HTTP, if this parameter is non-empty, then its value will
        replace any existing Content-Type header on the HTTP request.
        In the case of FTP and FILE, this parameter is ignored.
 @param aContentLength
        A value of -1 indicates that the length of the stream should be
        determined by calling the stream's |available| method.