This property specifies the folder where to store additional
data such as images and fonts. it is only used when exporting
documents to HTML. It is ignored otherwise.
If you set a relative path, it will be relative to the location
of the output path passed to Doc.Save(). If you set an absolute
path, the output html will contain absolute URLs.
If you are saving to a stream and specify a relative path, then
the path is relative to the current directory.
If you are saving to file and do not specify a folder, then a
folder with the same name as the output file will be created in the
directory where the output file is located.