re-instate float for attachment image

I have added float back in and included contain-floats to parent element .well
I haven't included fixed height or width to the attachment for 2 reasons
1. PDF thumbnails generated should be restrained to a max height and width
2. Icons for other attachment types should be the correct size and shoudln't need constraining using css

If either 1 or 2 are incorrect I'm happy to add height and width constraints