Check to see if a filename is valid
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 Syntax

        H_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

 Examples:

      Check that a file name is valid :

          lCheck := H_IsFileName( ".txt" )

      returns .F.

 Source: DOSFILE.PRG

 Author:  Martin Colloby