This is a generic problem where webkit does not properly return the name of a form control that has an image type.
Created attachment 27711 [details] patch to return alt tag on input file type
Created attachment 27713 [details] patch to return alt tag on input file type
Created attachment 27714 [details] patch to return alt tag on input file type
This was landed in <http://trac.webkit.org/changeset/41025>.