This indicates an attack attempt to exploit an Arbitrary File Upload vulnerability in Synology Photo Station.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application. A remote attacker may be able to exploit this to upload arbitrary files onto specific directories, leading to possible further attacks.
Photo Station before 6.7.3-3432 and 6.3-2967
Security Bypass: Remote attackers can bypass security features of vulnerable systems.
Refer to the vendor's advisory for updates: