Documents with external images are ignored

https://s3.amazonaws.com/uploads.hipchat.com/6795/13671/5rNNvSAE7MPZvhE/external_image.docx
How do you feel about making this a general utility function?

Also, I instead of doing a whitelist based on protocol prefix, could you just check whether the URI is internal and take the negative? An internal URI would start with `/` or not have a protocol at all.