JBIDE-17818 Visual Editor: Need to handle external html / htm files

@maxandersen @dgolovin @fbricon @alexeykazakov - I need +1 to push it into X branch
it is a bad smell if we rely solely on the filename extension to identify files. Why not attempt to get the sourcedocument no matter what the file the user is requesting to open and if you cannot get the model then handle that situation instead no matter the extension ?