I wouldn't bother with a constrain found set for this.
No need to show all records either.
Just set your script up to specify all the find criteria in the appropriate fields and then perform the find.
See this thread for examples of scripted finds: Scripted Find Examples
But if you did want to script such a constrain found set to filter down an existing found set, you could script it like this:
Enter Find mode  ----> clear the pause check box
Set Field [YourTable::FileTypeFieldHere ; $Type ]
Set Error Capture [on]
Perform Find