TeamForge has a small internal mapping of which icon is associated with which mimetype(s). This mapping is necessarily small, as a complete mapping would be exceedingly large, and almost always out of date. You can examine the mimetype that TeamForge is receiving by examining the document's attributes using our SOAP API. You can also override this mimetype info via the API by uploading a new version of the document (which can be the exact same document contents).
There is sometimes a generic icon for documents from certain applications. TeamForge accepts and stores the mimetype that the user's browser sends on the HTTP upload of the original document. As an example, let's examine the MS Word document. Some browsers will send strings like application/vnd.ms-word; others will send application/msword and others will send something completely different.