Personally, I'm not sure that's the best way to go about it. I'd just use the rmagick interface. http://rmagick.rubyforge.org/
With that you can get the width/height/type much easier. See these (slightly verbose) examples.
In particular, you could use something like
file.rewind img = Magick::Image.from_blob(file.read)[0]
Then width & height are available in img.columns and img.rows
Well, I intend this to make it onto RubyForge one day, but since you have a need 
Drop this into your RAILS_ROOT/lib and Enjoy! If you want the test/unit/image_size_test.rb, send me an offline email.
-Rob
# image_size.rb
One of these days I need to put everything like this into an anonymously accessible repository, but this will do for now.
-Rob
Rob Biedenharn http://agileconsultingllc.com Rob@AgileConsultingLLC.com
Can you send me an offline email (i.e., not through the mailing list or forum) that has both the JPEG file and the actual code you're using? I suspect that there's just a quirk in your JPEG file (I ran into a few oddities myself) that needs a new unit test to uncover. It looks like the end-of-file is being reached without finding the segment that contains size information.
-Rob
Rob Biedenharn http://agileconsultingllc.com Rob@AgileConsultingLLC.com Skype: rob.biedenharn