Customize file field display to show a download button?

Hello, how do I customize how a file is displayed? I am currently creating a site that allows people to download e-books formatted specifically for e-readers. How do I make it so the .mobi has a specific download button, and the html file has another.

I need more than just changing the CSS, because it has the words "FILENAME.htm" in it, and an icon. I want to change actual HTML code that displays the download button.

Also, if there where a way I could have the new download button as a display type in Content types » Book » Manage Display » formats that would be really neat.

http://books.tekkna.org/books/a-study-in-scarlet
Currently looks like:
http://oi48.tinypic.com/n4uxjp.jpg

Idea of what it should look like:
http://i48.tinypic.com/6fvbqx.png (beware of programmer art)