AnyAttach is a tweak that enables you to attach arbitrary files to your emails right from inside MobileMail.
It comes with a file and photo/video picker that is accessible via the paper clip button that will be added to the subject bar in the mail compose window.
Dropbox support is enabled by default. It can be disabled using Preferences.app -> AnyAttach
No icon will be added to your homescreen. Options can be configured from the Settings app.