Determining if a file is a virus based solely on its extension is unreliable and ineffective. File extensions (like .exe, .docx, .jpg) simply tell the operating system which application should open the file; they do not indicate the file's content or intent. Malicious actors frequently disguise viruses using harmless-looking extensions (e.g., renaming a dangerous program to "invoice.pdf.exe") or leverage common extensions associated with macros or scripts. A benign extension guarantees nothing about safety.

For example, a file named "report.txt" might actually be malicious executable code hidden by exploiting file display settings. Phishing emails often deliver viruses using compressed archives like .zip or .rar files containing malware, which have legitimate uses. Similarly, a legitimate .scr file (screen saver) can easily harbor malicious code. Attackers constantly invent new file types or misuse old ones to bypass simplistic detection methods.

Relying on extensions creates dangerous false confidence. Many harmful files use common, trusted extensions, while many files with unusual or suspicious extensions are perfectly safe. The only reliable way to determine if a file is a virus is through thorough scanning using reputable, up-to-date antivirus software before opening the file. Safe computing practices like not opening attachments from unknown senders are far more critical than monitoring extensions.