it was using bytes, but this seems wrong now that we did 31af8997a6
(XMLExportImport: Base_asXML and Folder_asXML return str, not bytes.,
2024-03-07).

use Image portal type to store image data. Using a Web Page never made
sense, it was working on python2 because the lax difference between
between text and bytes.

This test a svg contained in various portal types (Image, Web Page, )
with a png background image. The problem was with the background image.
This change does not affect the portal type containing the svg, only
the png background image.

This was based on libraries that were never ported to python3. If this
is still needed, it would have to be reimplemented with other newer
technologies.

some conversion (TextDocument) depend on the content type, so it's
necessary to first update the content type before trying to convert to
base format