I was interested in adding this feature because now I have some website members set up to send some newsletters on specific topics themselves. Just yesterday one of them asked about adding image attachments.
However I didn't anticipate it would be this much work to add the feature. So for now I'll ask them to upload the image to another place and get it through WYSIWYG

The new v10 media tag will handle this use case. Just upload to the library, then refresh it in a media tag. It does all the same rendering potential as attachments, without the unnecessarily complex security model.