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.
We're using ImageMagick 6.3.9-1 and your command works fine. Try explicitly setting the image format as in jpeg:humb_ff6a273d44106fa87753d6ed58640e8b.sys.