Quill is a web content editor, and Squire is an email editor. They are very similar, but slightly different - e.g. Squire supports quoting and changing quote levels, and knows how to handle attachments.
Not sure what's about either. Like many rich-text editors, it can handle embedded images, but the attachment stuff proper is handled in the surrounding application.
My mistake - I was editing an email at the time, and looking at the editor and roving over the UI, I thought "attachments, of course!", without actually checking to see whether or not that was part of the editor or not.
