This indicates an attack attempt against an Arbitrary File Upload vulnerability in WordPress Reflex Gallery 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 Reflex Gallery Plugin 3.1.3 and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.