I had several problems by downloading files attached to K2 items (Word and Excel file were corrupt). I have fixed this for my client by changing download link to direct link to files. No core file editing needed, just one line in template.
Please see the attached file for instructions how to do it!

Cheers for sharing this fix, worked a treat for me..

If anyone else has the same problem, give this one a go..

MY ERROR: IE7/8 item PDF attachments download size incorrect/file corrupt error, works fine in FF/Chrome