Only bind file upload callback for image assets
Only bind file upload callback for image assets

SectionAssetEditView.processForm was binding the image uploading handler
for all asset types, not just images.  This is wrong, so I added a guard
to check for the existance of an 'image' field on the form.  This seemed
more appropriate than checking the model type.

denverfoundation/storybase,denverfoundation/storybase,denverfoundation/storybase,denverfoundation/storybase