[exim] Attachment?

We have a server sending out Excel (.xls) attachment by email via local smtp
server (exim as MTA) relayed to our new ISP.

It turned out the .xls attachment was converted to a text file.

Did exim do this or probably the ISP do that (perhaps by antivirus program)?
How can I verify it from local smtp server (exim)?
Thanks.

Regards,
Benny