elm_cnp: adjust image type to real format

elm_cnp: adjust image type to real format

we should never say that we have image/png etc. if we do not have the
respective format. Otherwise we are just confusing the other client.

With this you can copy things from elm to thunderbird / libreoffice
writer / firefox / telegram / chromium.

Please note if you test this: Start *a new xorg session* it was
discovered that xorg clients tent to be totally confused if there was a
protocol error before, or some wrong mime type was delivered before,
thunderbird even likes to crash if this is not done.

This should at some point be extended to more image types.