To save a WordPad document with own file extension, use double quotation marks (") when saving the file. Follow the steps below:
1. Click the File menu and click Save as.
3. In the File name text box, type "filename.extension" and click Save.
For more information, please refer to the following solution in Microsoft's technical support website.
Article ID: Q253688
Last Modified: May 10, 2001
Title: How to Choose Your Own Extension Saving a Word Pad Document