To open the file in an application you will always have to export it to disk.
You can use the Get ( TemporaryPath ) in your script and select Automatically Open in the Export Field Contents step.
The temporary path is discarded when Filemaker quits.
Do you want to edit the file in the app and reinsert it back into Filemaker?
That is a bit more challenging to automate. You could export to Documents and drag & drop when you're done.
Easier than accessing the Temporary Path.