When updating a file attachment, it's likely that the user won't want to change

the caption. In this case, presenting a blank caption field will encourage them

to fill something in, resulting in different revisions having different

captions. This change makes it so that the field in the upload dialog is

pre-filled with the existing caption when updating.

Clicked "Add File" and saw the caption field was empty.
Clicked "Update" and saw the caption field was pre-filled with the old

  caption.