Futurebasic/Language/Reference/filesstring function

Function
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Description
Returns the file type of the last file returned by the ...  function, as a 4-character string. If the user clicked "cancel" in response to the last File Open dialog, or if the ...  function has never yet been executed, then the  function returns an empty (zero-length) string.

Note: In some cases it's useful to express the file type as a 4-byte long integer rather than as a string. Use the  function and the   function to convert between these two forms.