Post any defects you find in the released or beta versions of the ImageMagick software here. Include the ImageMagick version, OS, and any command-line required to reproduce the problem. Got a patch for a bug? Post it here.
From the mogrify manual:
"By default, the image format of `file' is determined by its magic number."
$ file 982434113.jpg
982434113.jpg: JPEG image data, JFIF standard 1.02
$ mogrify -resize "40x40" 982434113.jpg
# OK
$ mv 982434113.jpg 982434113.tmp
$ mogrify -resize "40x40" 982434113.tmp
mogrify: no encode delegate for this image format `982434113.tmp' @ constitute.c/WriteImage/1157.