As a fallback, you can use Google to search ET for attachments of a given type. For example, the search string: This search returns hits to threads that have excel spreadsheets in them. It's not perfect because it's not by date (or perfectly up-to-date), but its extremely fast and its something.
Maybe you could change it so only threads where the first post contains an attachment shows a paperclip. The current behavior is not really useful.
Not sure how useful that change would be. Wouldn't that setup be far too restrictive, too narrow? Threads with an attachment after the very first post have got to be a small fraction of all threads with an attachment. The current setup is probably best used as a backup for those (rare) cases when a targeted search -- keywords, file type, dates, whether here or through Google -- doesn't deliver.