Origami allows user to upload any file as sample image

Node version: 8.3.0

npm version: 5.5.1

Operating system: Debian Jesse

Command line used: bash

Steps to reproduce:
1. Go to any origami demo.
2. Try uploading text files and python scripts.

BONUS:
If you are the admin of an origami demo, you can do the above steps and upload malicious files and can even run JavaScript on the user's browser.

UPDATE:
Have written some code to fix this for the time being. Will make a PR in the morning.

Start by reproducing the upload in an Origami demo with a text file and a Python script, then trace the demo's sample-image upload and browser handling. Done means non-image files are rejected and uploaded content cannot execute JavaScript in users' browsers; the payload does not name a file or test to run.

Written by the indexing model from the issue text.