Fix Firefox file upload error in Fashion demo

[ckadner]

These changes are adopted from https://github.com/IBM/MAX-Image-Caption-Generator-Web-App/blob/master/static/js/webapp.js

IBM/MAX-Image-Caption-Generator-Web-App#22
IBM/MAX-Image-Caption-Generator-Web-App#37
IBM/MAX-Image-Caption-Generator-Web-App#40

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[Tomcli]

LGTM