Notes about the 521 file extension:
To open this file type, a required software must be installed. Without it you only see a Windows message "The following file cannot be opened
" (Windows 7) or "How do you want to open this file?
" (Windows 10) if you double-click the file. There click on "More apps"
and choose a suitable software to view such files on your PC or from email attachments.
The 521 file type is primarily associated with 'Murray State University Trojan Ring Configuration File'. The Trojan Ring is a ring of students. There are 7,000 PC's, and 13,000 users. All of them connect to the Trojan Ring, and it acts as a firewall, because the common Trojans here look for IPs with vulnerable ports, and the ring bombards it with IPs and vulnerable ports, all of which are static and bgus.
Detailed information for file extension 521:
Primary association: Murray State University Trojan Ring Configuration File
To change file associations press Windows Key
and then Apps and Features
> Default apps
> Choose default apps by file type
Invalid file associations could be the result of underlying issues within your Windows PC.