Image Filename Issue

[georgeh]

Steps to reproduce:

1. Insert -> Image -> By URL (this preserves the filename)
2. Put in a URL with an unsupported character
   • + is known bad, there are probably others
   • https://george.hotelling.net/images/9Cze+.jpg can work as a test file
3. Save draft

What actually happens:

• An alert is displayed with an error saying "File is empty. Please upload something more substantial"
• The image is not uploaded
• The draft has an empty image tag

Test document: https://docs.google.com/document/d/1ah1ClnNfy02mlXcnSPp3P7gm_PMta1q5tAsi-gu_OYM/edit