Where JFIF came from
JFIF is the 1992 wrapper specification that made raw JPEG data into an actual interchangeable file — it defines the density and thumbnail headers every JPEG carries. The .jfif extension mostly appears by accident: certain Windows and Outlook versions save downloaded JPEGs with it, confusing software that matches on extension rather than content.
The technical picture
Compression: Lossy DCT — identical to JPG. Transparency: None. Animation: Not supported. Colour: 8-bit per channel, sRGB. Typical file size relative to WebP: 25-35% larger than WebP at matched quality.
Where it opens
Universal. This is usually the deciding factor when choosing between JFIF and WebP — WebP renders in every browser but is uneven in desktop software, while JFIF has a very different support profile.
Converting JFIF to WebP
Converting JFIF to WebP both cuts the file size and gets you out of an extension that a surprising amount of software refuses to recognise. Lossy to lossy, so a little generational loss is added.
Converting WebP to JFIF
Rarely requested — JPG is the same format with the expected extension.. Expect the JFIF output to be 25-35% larger than WebP at matched quality — that difference is the price of getting a format the destination will actually accept.