The external helper app service is used for finding and launching
platform specific external applications for a given mime content type.
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.
| aMimeContentType | The content type of the incoming data |
| aRequest | The request corresponding to the incoming data |
| aContentContext | Used in processing content document refresh headers after target content is downloaded. Note in e10s land this is likely a CPOW that points to a window in the child process. |
| aForceSave | True to always save this content to disk, regardless of nsIMIMEInfo and other such influences. |
| aWindowContext | Used in parenting helper app dialogs, usually points to the parent browser window. This parameter may be null, in which case dialogs will be parented to aContentContext. |
| A nsIStreamListener which the caller should pump the data into. |
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.