Recognize .svg.gz as images

[kno10]

Using a .svg.gz file will be rendered using an <embed tag instead of an <img tag.
This is caused by file yielding a mime type of application/gzip.
But IMHO it is desirable to use .svg.gz because of the high compression ratio achievable on this format.

[jgm]

I'll change this - but for now .svgz will work.