Applescript - opening file inside app bundle
You can use path to resource to get the pathname of the  file that's in an App's Resource folder.

Example AppleScript code:

set resourceName to "Exolon.tzx"

set filePathName to quoted form of POSIX path of (path to resource resourceName) as text

tell application "Fuse"
        open POSIX file filePathName
end tell




Note: The example AppleScript code is just that and does not contain any error handling as may be appropriate. The onus is upon the user to add any error handling as may be appropriate, needed or wanted. Have a look at the try statement and error statement in the AppleScript Language Guide. See also, Working with Errors.