.PLAIN File Extension
What is a PLAIN file?
A PLAIN file is a plain text file that can be opened by any text editor. It contains plain text with no special formatting. PLAIN files may be used for a variety of purposes, such as storing a plain text version of a document, a copy of an email in HTML, or a receipt from a business.
If you receive a PLAIN file you can use any number of free text editors to open the file. Notepad and Wordpad are bundled with Windows, while TextEdit is bundled with macOS.
NOTE: You can change the ".plain" extension to ".txt" to make it a .TXT file.