What a JFIF File Actually Is
JFIF stands for JPEG File Interchange Format, and it is not a different image type: it's the standard wrapper that ordinary JPEG data has shipped in since the early nineties. Nearly every .jpg on your disk is technically a JFIF file. The only thing unusual about a file named .jfif is the name, which Windows started handing out for some web downloads a few years back.
The trouble is that plenty of upload forms filter by extension instead of content, so a perfectly good photo gets rejected for its last five characters. Converting here re-encodes the image through your browser's JPEG encoder and hands it back with a .jpg extension that passes every filter.
Two honest caveats
First, re-encoding a JPEG is a lossy step, which is why the quality slider sits at 85 by default; push it to 95 or higher if the file is precious, and the readout will show the size cost. Second, if you only need the name changed, renaming the file from .jfif to .jpg in your file manager also works, because the bytes were JPEG all along. This page exists for the times a rename feels risky, a form still refuses, or the file is one of a batch you're cleaning up anyway.
Want the photo lighter while you're at it? Lower the slider and check the green number, or take the result to the resizer if its pixel size is the real problem.