Why it happens
WebP was designed in 2010 as a browser delivery format, and browsers adopted it accordingly. Desktop applications had no reason to hurry, and many still have not: Photoshop gained support only in version 23.2 (2022), Microsoft Office in the 2021 release, and Outlook still will not render one inline in an email body.
Why you have one at all
Because the modern web serves WebP by default. WordPress converts uploads to it, Shopify serves it, Next.js generates it, and Cloudflare, Cloudinary and imgix all emit it automatically based on the browser's Accept header. Right-clicking and saving gets you whatever was served — which is now usually a .webp.
What does open WebP
Every current browser, macOS Preview and Quick Look, Windows Photos, GIMP, Affinity Photo, Photoshop 23.2+, Office 2021+, Figma and most modern phone galleries. If your software is a few years old, that is usually the deciding factor.
Renaming it to .jpg does not work
A common suggestion online, and it does not help. The extension is a label; the bytes inside are still VP8-encoded. Software that reads the file header will reject it, and software that trusts the extension will produce garbage. The file has to be genuinely re-encoded.
The fix
Convert it. WebP to JPG is the safe universal answer, and WebP to PNG is the one to pick if the image has transparency you need to keep. Both take a few seconds and need nothing installed.