Do you have a filter set up that's sending emails that include "DVD" or "VHS" or some other non-word directly to your Trash? Unfortunately, short strings of letters like this are often also found in the encoding for attachments. As it processes masses of data, Yahoo! Mail sees the string of letters, matches them with your filter, and sends them to the Trash.

Take a look at your filters: Do you think you have one or two that might be doing this? If so, the best way to start getting the attachments again is to remove these filters.