Added another option for the file field that will utilize the Media workflow introduced in WordPress 3.5.

This is a basic implementation that integrates the WordPress 3.5 Media workflow, but there are still some rough edges. Primarily the option should not be available unless you're running 3.5+ but I didn't want to jack up the array declaration in `file.php` in an effort to get some feedback on how to best handle that.

Further, integration with file type/subtype needs to be mapped to either WordPress mime types or something manual, right now there's no filter on the modal.

Last but not least is the fact that out of the box the Media workflow in 3.5 supports either 'single' or 'multiple' files as opposed to a specific limit on the number of files you can pick, so right now the only thing taken into consideration is whether or not the file limit is `1`. A potential solution would be to simply stop appending files once the limit got reached, but there would be no visual feedback in the Media modal as you were selecting files. Something to mull over.