JPEG and JPG are two spellings of one format. Drop the files, choose the ending you need, and get the same bytes back under the new name, with a SHA-256 checksum proving nothing changed.
Why no conversion happens
The three-letter .jpg comes from old file systems that allowed only three characters after the dot. The format inside is identical. Re-encoding a JPEG to change its name would lose quality for nothing, so this mode does not decode or encode anything: the output is the input's bytes under a new name.
Each file's SHA-256 checksum is computed in this tab for the file you dropped and for the file offered, and a file is only offered when the two match. Several files go into a ZIP with stored (uncompressed) entries, which keeps every byte as it was.
When the file is not really a JPEG
Sometimes a file named .jpeg is a PNG or WebP inside. Renaming it would make a file that lies about its format, so it is flagged instead, with an option to convert it to a real JPEG.
