yank-media filters out image formats that are not supported by emacs.

To do that is passes the second part of the mime type (png, jpeg, etc.)
to (image-type-available-p).  This is ok for other image types, but not
for SVG, because its mime type is image/svg+xml. For example, InkScape
(a free vector drawing program) uses image/svg+xml to copy vector data
into the clipbard.

This patch fixes that filtering. Also changes the function
mailcap-mime-type-to-extension, used by org-mode to choose an extension
for the file it saves. It nees to be svg, not svg+xml.