How about adding the doc types as classes?

If you know what document types (or extensions) Google has, how about using those extensions as CSS classes?

Here's an example:
<a href="https://docs.google.com/feeds/download/documents/Export?docID=...&export..." class="gdocs-download doc new-window" target="_blank">doc</a>

This way, someone can theme the correct icons into the links' backgrounds.