This indicates an attack attempt against an Arbitrary File Upload vulnerability in WordPress Drag and Drop Multiple File Upload Contact Form 7 Plugin.
The vulnerability is due to insufficient sanitizing of user supplied inputs when handling a crafted file upload. It allows a remote attacker to upload an arbitrary file onto vulnerable systems via a crafted HTTP request.
WordPress Drag and Drop Multi File Upload - Contact Form 7 Plugin for versions prior to 1.3.4
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.