Get the top-class for a file: e.g. application/xml for image/svg+xml

[Daniel Leidert <daniel leidert spam gmx net>]

Hello,

I currently try to figure out, how to get the top-class of a file. If I
have e.g. a DocBook XML and a SVG file, is there a function that returns
application/xml for both? I do not find a function in the docs, that
does what I need.

Or is there some other way to get this result?

Regards, Daniel