A user is able to extend the already existing list of some known files extensions containing only text. ASCII mode attempts to preserve formatting across operative systems without altering them.
It’s recommended that binary file extensions should not added here otherwise it corrupts the binary files.
Commands are; 
New Entry | 
Delete Entry | 
Edit Entry
The syntax for adding a file extension to this list is to use .ext
For example with the file index.html, i.e. .html You can also add the exact filename, i.e. index.html