Use a file extension as a clue, not proof

A file extension is a clue about a file's intended format. It is not proof of its contents, safety or origin.

Enable the display of full filenames using your operating system's official settings. This helps reveal confusing names and unexpected final extensions. Be cautious with unsolicited attachments, even when the visible name resembles a familiar document. Confirm an unexpected file with the sender through a known channel before opening it.

Renaming is not conversion

Changing the letters after the dot does not reliably transform one format into another. Use a trusted application's export or conversion function when you need a different format, and keep the original. Check that the resulting file opens and retains the information you require.

For an unfamiliar format, consult the software provider's documentation. Avoid installing a random viewer promoted by a download page simply because it claims to open everything. Keep the system and relevant security tools updated through official channels.

If a file behaves unexpectedly, stop rather than trying several increasingly permissive ways to run it. Follow your organisation's reporting process when work material is involved. Understanding the extension helps you ask the right question about a file. It does not remove the need to consider where the file came from and why you received it.