Would be better to read into the file to identify it instead of rely 
on extension. (1st 8 bytes) Ubuntu/nautilis is using the extension... :(
That way any weirdly named files are access properly without having to rename them. (CD/RO)




Other information:
function is_filebinary(gzipfile : string) : boolean;

var tb : boolean;

    maxtoread, i : byte;

    b : array [1..10] of byte;
    gzfile : file of byte;

begin

     for i := 1 to 10 do b[i] := 0;

     tb := false;

     assign(gzfile, gzipfile);

     {$I-} reset(gzfile); {$I+}

     if ioresult <> 0 then exit;

     if filesize(gzfile) >= 10 then maxtoread := 10

        else maxtoread := filesize(gzfile);

     for i := 1 to maxtoread do

         begin

              read(gzfile, b[i]);

         end;

     close(gzfile);

{     exit;}

     for i := 1 to maxtoread do

         begin

              if (b[i] < 32) or (b[i] > 126) then

                 begin

                      if (b[i] = 10) or (b[i] = 13) then

                         else tb := true;

                 end;

         end;

     is_filebinary := tb;

end;