Check to see if a filename is valid
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 Syntax

        GT_IsFileName( cFile )

 Arguments:

      cFile - File name

 Returns:

      .T. if cFile is a valid DOS file name
      .F. otherwise

 Description:

      Checks to see that the given string passes the tests for validity
      as a DOS file.  Does not allow drive path or extension to be passed.

 Examples:

      Check that a file name is valid :

          lCheck := GT_IsFileName( ".txt" )

      returns .F.

 Source: DOSFILE.PRG

 Author:  Martin Colloby