You should be allowed to view attachments. Could you show me a screenshot of that error message? I've checked all the settings, and you should be able to see them. Even if you weren't logged in, you should still see them. But a screenshot will help me track down where the problem is. Also, in case it's only SVG attachments, I've attached PNG below. Can you see it?
Because, as we've discussed, SVG support can vary quite a lot, Inkscape would not be able to guess which features are or aren't supported, in whatever place where you will be using your SVG file. So no, there's no such setting that can do that.
While more and more browsers are gaining more and more support for SVG, still the best advice is to use images in raster formats, for display on the internet, if you want everyone to be able to see it. Of course in certain situations, especially like for an interactive page or image, or other unique SVG features, you'd want to use SVG. But for something that you want to always be showing, raster should be used.
PS - Very nice drawing, by the way! Very professional look
Edit -- Oh, do you mean in the message Preview? Yes, it does show that in the Preview, and I don't know why. But when you look at your message after it's posted, you can see the attachment there, right?