Merge some file writing extension checks from OpenJK.

Thanks Ensiform.
https://github.com/JACoders/OpenJK/commit/05928a57f9e4aae15a3bd0
https://github.com/JACoders/OpenJK/commit/ef124fd0fc48af164581176

code/client/cl_console.c

@@ -191,6 +191,12 @@ void Con_Dump_f (void)
 	Q_strncpyz( filename, Cmd_Argv( 1 ), sizeof( filename ) );
 	COM_DefaultExtension( filename, sizeof( filename ), ".txt" );
 
+	if (!COM_CompareExtension(filename, ".txt"))
+	{
+		Com_Printf("Con_Dump_f: Only the \".txt\" extension is supported by this command!\n");
+		return;
+	}
+
 	f = FS_FOpenFileWrite( filename );
 	if (!f)
 	{