Content-type mismatch warning on attached pdf document

[vantky21415]

This attachment is suspicious because its type doesn't match the type declared in the message. If you do not trust the sender, you shouldn't open it in the browser because it may contain malicious contents.
Expected: application/pdf (.pdf); found: application/x-empty

so, i can't open, and  if download, it bacomes zero byte file.

However, attached pdf document is ture, and application like thundirbird, not webmail,  i can open, and see.

sometimes happen. especially mixed attachemend pdf and jpg.

How can i solve this?