Adding external SVG files is more and more populare in resent projects and it will spread the use cases and applications were it can be used.
The Default setting for a 1und1 Windows Hosting packacke don't support SVG-files and show the 404 error page for not found. 
One reason why 1und1 SVG files not added in the default web.config files is SVGs can includes CSS and
Use the mimeMap element between the staticContent and put in in the syste.webServer decleartion.
Remove the extension first, before adding it again. Some errors occurred because each extension can be added once!
... <remove fileExtension=".svg"/> <mimeMap fileExtension=".svg" mimeType="image/svg+xml" /> <remove fileExtension=".svgz"/> <mimeMap fileExtension=".svgz" mimeType="image/svg+xml" /> ...
Here a sample web.config which you can put on the web-server.
 Already configured MIME types for 1und1 Windows Hosting:
1&1 Help Center - Specify Additional MIME Types
 Blackhat USA 2014, Rennie DeGraaf (PDF): "SVG: EXPLOITING BROWSERS WITHOUT IMAGE PARSING BUGS"