Sanitize guid on save and display. Sanitize mime type on save. Don't allow...

Sanitize guid on save and display. Sanitize mime type on save. Don't allow changing mime type via edit form handlers. Protect hidden meta.

git-svn-id: https://core.svn.wordpress.org/trunk@17994 1a063a9b-81f0-0310-95a4-ce76da25c4cd