Convert JFIF to JPG

JFIF is the file layout inside almost every JPEG ever saved. A file ending in .jfif is an ordinary JPEG that was given an unusual extension, most often by Windows when you save a picture from a browser. Change the ending to .jpg and it opens everywhere, with no loss at all.

Several files, or renaming is not an option?

Drop them on the image converter and choose JPEG. Each file is re-saved as a .jpg on your device — nothing is uploaded. This re-encodes the picture once; at the default high quality the difference is not visible, and it lets you shrink the files or turn them into PNG or WebP at the same time.

Why Windows saves .jfif

Windows keeps a table saying which extension goes with each file type, and on many PCs the entry for JPEG images says .jfif. Browsers ask Windows for that extension when you save a picture, so they get .jfif. The entry is in the registry at HKEY_CLASSES_ROOT\MIME\Database\Content Type\image/jpeg, value Extension; setting it to .jpg stops it. Editing the registry is at your own risk — renaming or converting the files is the safe route.