Apple Macintosh PowerBook 150 - Manual (Page 146)

3 Click the Add button.
A dialog box appears in which you can specify a DOS suffix and its
corresponding Macintosh application program and document type.
4 Type a three-letter DOS suffix in the DOS Suffix box.
To specify which Macintosh program will open DOS documents with no
suffix, click inside the box to set the insertion point, but leave it blank.
138 Appendix B