I'm looking for statistics that show which file types (.doc, xls, pdf,
etc) are most frequently sent as attachments in emails. I'm working on a
project that processes incoming emails, and need to know the expected
frequency for various filetypes.

I'd like as detailed a list as possible, and not just a top 10 list. 
Clearly, word/xls will dominate the numbers, but the less frequently
occuring ones are important as well.

If it is not possible to get the email attachment data, then a ranking
of filetypes in the business marketplace would suffice.