This indicates an attack attempt against an arbitrary File Upload vulnerability in Wordpress page-flip-image-gallery plugins.
The vulnerability is due to insufficient input validation in the application when handling an unauthenticated file upload. It allows a remote attacker to upload an arbitrary file onto vulnerable systems.
Wordpress page-flip-image-gallery plugins
System Compromise: Remote attackers can gain control of vulnerable systems.
Currently we are unaware of any vendor supplied patch for this issue.