This indicates an attack attempt against an Arbitrary File Upload vulnerability in WordPress download-manager plugin.
The vulnerability is due to insufficient sanitizing of user supplied inputs when handling an HTTP request. It allows a remote attacker to upload an arbitrary file onto vulnerable systems via a crafted HTTP request.
Wordpress Download Manager version 2.7.4 and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.