PNG signature from the PNG specification, section 5.2, eight bytes:
89 50 4E 47 0D 0A 1A 0A

benign-mislabeled.jpg contains only those eight bytes.
It has no IHDR chunk and no IEND chunk, so it is not a complete PNG datastream.
No image parser was run. The filename and the declared type are claims.

Worked card
filename: note.jpg
declared: image/jpeg
bytes: 68 65 6C 6C 6F
ASCII from Stage 04: hello
The bytes are letters.

Activity card
filename: badge.jpg
declared: image/jpeg
bytes: 64 65 73 6B
ASCII: desk
The bytes are letters.

Field card
filename: image.jpg
declared: image/jpeg
bytes: 89 50 4E 47 0D 0A 1A 0A
Compare these bytes with the reference signature and state what format they suggest. A signature match alone does not establish a complete valid image.