nsIExternalHelperAppService in Firefox 3.5

 Returns true if data from a URL with this extension combination
 is to be decoded from aEncodingType prior to saving or passing
 off to helper apps, false otherwise.

 Binds an external helper application to a stream listener. The caller
 should pump data into the returned stream listener. When the OnStopRequest
 is issued, the stream listener implementation will launch the helper app
 with this data.
 @param aMimeContentType The content type of the incoming data
 @param aRequest The request corresponding to the incoming data
 @param aWindowContext Use GetInterface to retrieve properties like the
                       dom window or parent window...
                       The service might need this in order to bring up dialogs.
 @param aForceSave True to always save this content to disk, regardless of
                   nsIMIMEInfo and other such influences.
 @return A nsIStreamListener which the caller should pump the data into.