Give a file name a default extension.
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 Syntax

        GT_DefExt(<cFileName>,[<cDefExt>]) --> cNewFileName

 Arguments:

      <cFileName> is the file name to be checked.

      <cDefExt> is an optional character paramater which is the
      extension to add to the file. This can be in the format of
      `.xxx' or just `xxx'. If not supplied the default is "".

 Returns:

      The file name as it was if it had an extension, or the file name
      with the default extension if it didn't have one.

 Description:

      GT_DefExt() can be used to supply a default extension to a file.
      This can be of use in file i/o functions or for tidying up user
      input.

 Examples:

      // Get a file name form the user and default the extension.

      cFileName := space(12)
      @ 10,10 say "File To Open:" get cFileName picture "!!!!!!!!!!!!"
      read
      cFileName := GT_DefExt(cFileName,"DBF")

 Source: DEFEXT.PRG

 Author:  Dave Pearson