I use the following method . . .
For instace a PDF file
Add 2 icons to the folder ....../images/attach/
The names should be pdf_thumb.gif and pdf.gif
The first one is a question mark which is used in the Manage Attachements window and is the same for all non-image attachements. The second one is a file icon, I use the adobe reader logo.
I use these two:

and
To do this for RAR the file names would be rar_thumb.gif and rar.gif. The first one would be the question mark again (atleast afaik that is the standard VBulletin method) and the second one would be a rar icon.