Support for progressive jpegs via onprogress event

[bompo]

Loading progressive jpegs is possible but not really useful without a onprogress event. Would it be possible to add support for a onprogress event? This should deliver the partly loaded jpeg.
Note that this would require to load the binary data with overrideMimeType('text/plain; charset=x-user-defined') instead of arraybuffers. Otherwise you get a DOM exception for incomplete arraybuffers.