This is due to the way Microsoft implemented the Save As dialog box: If you supply an extension it doesn’t recognize (including none), it either appends the default extension, if the filter is “*.*”, or the first extension in the list for the selected filter. This behavior is designed to enforce the strong association between extensions and applications in Windows, but it does get in the way at times. One of the most irritating aspects of this is that it can create files with names like FILE.XYZ.TXT, when you type FILE.XYZ.
To allow a file to be saved with an arbitrary extension, delete the default extension from the File page of the Preferences dialog box, and select “*.*” from the “Save as Type” list on the Save As dialog box. Alternatively, you can force it to accept exactly what you type by putting quotes around the name.