Why WebP files cause trouble
WebP is a modern image format that Google created to make websites load faster, so browsers now save many pictures as .webp. Phones and browsers can show them, but plenty of software still cannot: older photo editors, some Office and print tools, and many upload forms on government, bank and job portals that accept only JPG or PNG. Converting to JPG fixes the 'file type not supported' error.
Quality and file size
WebP is usually smaller than JPG for the same quality, so the JPG may be somewhat larger than the WebP you started with. At 90% quality the picture looks the same as the original. If a portal has a strict size limit, lower the quality to 70 or 80%, or use the image compressor to hit an exact size such as 100 KB.
Transparent WebP images
WebP can have see-through areas, like PNG, but JPG cannot. Transparent parts are filled with white by default, and you can pick another colour so a logo or sticker matches the page it is going on. If you need to keep the transparency, convert to PNG instead.
Private and free
The conversion uses your browser's own image engine. Your pictures are decoded and saved as JPG in memory on your phone or computer and never sent to a server. There is no account, no daily limit and no watermark.