$ mogrify -format jpg -- *

$ mogrify -resize 900x900 -- *
mogrify: insufficient image data in file `109.jpg' @ error/jpeg.c/ReadJPEGImage/1039.
mogrify: no decode delegate for this image format `' @ error/constitute.c/ReadImage/501.
mogrify: no decode delegate for this image format `' @ error/constitute.c/ReadImage/501.
mogrify: no decode delegate for this image format `' @ error/constitute.c/ReadImage/501.
mogrify: no decode delegate for this image format `' @ error/constitute.c/ReadImage/501.

$ file * | grep -Po "^d*: *K[^:,]*(?=,)" | sort | uniq
ASCII text
GIF image data
JPEG image data
PNG image data