Check a file name is valid
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 Syntax

         GT_IsFile(<oGet|cFile>,[<cType>],[<lExists>])

 Arguments:

       <oGet|cFile> Get object to check the file name or
       just pass a name.

       <lWildCards> Matching spec.

       <lExists> Must the file exist ?

 Returns:

       .T. / .F. based on the success.

 Description:

       Check that a file name is valid and optionally if it
       exists.

 Examples:

 Source: GT_ISFIL.PRG

 Author:  Martin Bryant