Email extensions converted to .bin

[Supine]

I have a gmail account and whenever I receive emails from one particular person with an attached file the attachment is renamed to something like 11e1a67a2fc.bin

I then have to download the file, guess the original extension and rename it. This only happens with this one person.

Anyone know what might cause this? I suspect an anti-virus program or something similar may be renaming the attachments on his side.