MINOR: Fix broken 'Insert From CMS' layout

An update to UploadField styles https://github.com/silverstripe/silverstripe-framework/commit/46a5ee7d1de7fecd574b091377abad5a56326126 broke the layout of this form a bit

![silverstripe_-_edit_page](https://cloud.githubusercontent.com/assets/1079425/5057744/21bdb6c2-6d22-11e4-9b13-de3f5812a8ed.png)