Bugfix 1019925 - Images with Data-URI's now show

Either as a comment here or at its usage point I think it would be handy for us to mention that we intentionally do not care about:

1. The MIME type (well, media type if https://developer.mozilla.org/en-US/docs/Web/HTTP/data_URIs is believed) since the <img> tag already restricts what it will load/etc. and so can't be tricked into doing something nefarious.  Actually, I am a little concerned about this in case SVG can be loaded and if it can reference external resources, etc.  I'll needinfo/feedback :freddyb on that and put more detail on the bug.

1. The actual encoding type/content of the encoding.  This assumes that there are either no MIME-type issues like the SVG thing above and/or the MIME type constraint is sufficient and can't be defeated via us doing sniffing or something.