This indicates an attack attempt against an Arbitrary File Upload vulnerability in WordPress Ninja Forms Plugin.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application while handling maliciously crafted file upload request. A remote attacker can exploit this to execute arbitrary code within context of the affected application via a crafted file upload request
WordPress Ninja Forms 2.9.36 to 2.9.42
System Compromise: Remote attackers can execute arbitrary code in the context of the affected user
Apply the most recent upgrade or patch from the vendor.