The antivirus product can be tricked into not scanning potentially malicious files.
The remote host is running the Kaspersky antivirus client. This version of Kaspersky is vulnerable to a flaw where file scanning can be bypassed by passing malformed MIME-encoded requests. An attacker exploiting this flaw would be able to send malicious files through the antivirus product without being detected.
Upgrade or patch according to vendor recommendations.