fdo#81113 always run the actual detection

The old code only run detection if an UCBContent property was passed.
That means that any time the function got only InputStream, it claimed
that it was a Keynote presentation.

Change-Id: I377828229e7e95384257bde247612d7768307581
make keynote detection a bit more robust

Change-Id: I947bc5867a6b61a923d47cad48d04ab5fc2769c9