I ran into this today. This isn’t WP specific, but whatever. I’m sure I’ll have to do this again at some point.
Basically, .SVG files are Scalable Vector Graphics. A .SVGZ is a compressed version of it. It is a really good type of file format on the web for things like logos and icons.
I setup a development site for a client and the .SVGZ files weren’t showing up. To fix it I just had to drop in two lines in a .htaccess file.
AddType image/svg+xml svg svgz AddEncoding gzip svgz