Consider fixing user mistakes and/or adding unit tests for $preferred_ext in wp_mime_type_icon()

[sabernhardt]

Follow-up to ticket:31352#comment:54

The $preferred_ext value for wp_mime_type_icon() begins with a period, and developers might miss that. The function could check the first character and add the dot if it does not exist there. The function also could ensure that the letters are lowercase.

For unit tests, the test_wp_mime_type_icon_video test in post/attachments.php checks the default $preferred_ext parameter value. Adding at least one more test with a specific $preferred_ext could help.