I bet that you have once created an asset, with a doctype filter, uploaded a file as an attribute, and noticed that the filetype and mimetype were not getting pulled. You checked the file once again to see that it had an extension and was actually not corrupted either. Everything seemed ok. You edited the asset. Re-saved it. Shook your head. Sweated a bit. Checked the asset definition to make sure that the doctype filter was still associated with the definition. Checked the doctype filter's attribute names. Well, you never know... a typo here and there... Nope. Nothing. You just don't know...