An admin can add a new custom file type

1) As an administrator, navigate to Site administration>Server>File types
2) Scroll down to the bottom of the list and click to add a new file type.
3) On the ‘Add a new custom file type’ screen, set extension as ‘mobi’, MIME type as ‘application/x-mobipocket-ebook’, description type as custom and the custom description as ‘Kindle ebook’. For File icon, choose ‘document’.
4) Click ‘Save changes’. The page will redirect to the custom file type listing page. Verify that the mobi file type is now listed.
5) Now in a course, upload a file with the extension .mobi. (You don't need an actual .mobi file; you can rename a .txt file instead)
6) Expand the Appearance section and click the Show type box, leaving other settings as default.
7) Click ‘Save and return to course’ and verify the icon for your newly created activity is the standard Moodle document icon.
8) Verify that ‘Kindle ebook’ is shown as the file type description.