We spent an absolutely ridiculous amount of time making drag-and-drop work across browsers in the pure-website version of Inky (http://inky.com). Its absurd difficultly was one of the reasons we migrated to an installed app model for our first release.
Making a "real" mail client that runs in a web browser is incredibly difficult. Basic things you take for granted when writing a desktop app, such as being able to copy text to the clipboard, or -- seriously -- being able to position the cursor in the editor window below the quoted text for "bottom posting", are a huge, grinding pain in browsers. And don't get me started on key bindings. Keyboard handling in browsers is still a comedy of errors.
It's gotten a lot better over the last few years, but it's still really hard to mimic some standard desktop app functionality using pure HTML/CSS/JS.
It doesn't mention what to do if the browser doesn't support FormData. What I had to do at Grooveshark is just force the user not to be able to drag/drop if they don't also have FormData.
