remove use of deprecated built-in file upload extension from demo
remove use of deprecated built-in file upload extension from demo

martijnvermaat/plexus-form,AppliedMathematicsANU/plexus-form,martijnvermaat/plexus-form,ismaelga/plexus-form,MaxKramnik/plexus-form,little-arhat/llexus-form,AppliedMathematicsANU/plexus-form,little-arhat/llexus-form,MaxKramnik/plexus-form,little-arhat/llexus-form,ismaelga/plexus-form,ashwinrayaprolu1984/plexus-form,MaxKramnik/plexus-form,martijnvermaat/plexus-form,joshgagnon/react-json-editor,ismaelga/plexus-form,ashwinrayaprolu1984/plexus-form,ashwinrayaprolu1984/plexus-form,AppliedMathematicsANU/plexus-form,joshgagnon/react-json-editor,joshgagnon/react-json-editor