HOWTO: edit mime types and its icons

First, install "file types editor" from ubuntu repository.

Run "file types editor" and press "new"

In general tab, select proper category, then enter name (e.g. text/x-mytype)

Select default icon. (e.g. ~/.icons/(theme)/scalable/mimetypes/text-x-mytype.png)

Go to filenames tab, and press add button.

enter "*.aaa", where aaa is file extension.

re-login.