- Allow image, pdf-image, or custom file types
- Global black list extensions to prevent unsafe file types
- Custom file upload is white listed only

# Conflicts:
#	AUTHORS
#	openassessment/templates/openassessmentblock/edit/oa_edit.html
#	openassessment/xblock/static/js/openassessment-lms.min.js
#	openassessment/xblock/static/js/openassessment-studio.min.js
#	openassessment/xblock/static/js/spec/lms/oa_response.js
#	openassessment/xblock/static/js/src/lms/oa_base.js
#	openassessment/xblock/static/js/src/lms/oa_response.js
#	openassessment/xblock/static/js/src/oa_server.js
#	openassessment/xblock/static/js/src/studio/oa_edit.js
#	openassessment/xblock/static/js/src/studio/oa_edit_settings.js
#	test/acceptance/tests.py