Hi Toby - thanks for this plugin - very simple and effective.
I've got an interesting little niggle for you tho' - not a showstopper by any means..
When working with a draft post with PDF links in I view the post and the icons are not displayed. No errors, anchor element picks up two new mlti classes. I checked to see if it was the theme by switching to asynchronous but this replaced the hyperlink with no file information.
When I publish file all is well and the icons work fine in synchronous and asynchrounous modes