Automatically change extension with syntax highlighting selection

[I7T5]

Problem

I use txt all the time. When I don't add the file extension to my document name, CotEditor saves it as a document with no extension, which is not super convenient.

Solution

Automatically change extension with syntax highlighting selection upon last save.

Alternatives

Manually adding extensions.

[1024jp]

This request is based on the same issue as #1542 and this will be solved in the next version CotEditor 4.8.5.