See: http://www.webgui.org/webgui/dev/discuss/thingy-specs-updated-version/6#id1p1YPDC0wFoqNJglRd6A2g
I sent a message along with an attachment and it showed up empty.
The assetId for this is 1p1YPDC0wFoqNJglRd6A2g and it isn't empty, just has HTML that doesn't display anything. The storageId is 5AX1wmTJlHe6L48B_D4JrQ but I'm still tracking down what that points to so I can see what happened to the attachment.
This is due to how the system picks parts out of the MIME messages. I have a pretty good idea how to fix it and have a decent test case for it.
Graham, you rule. If you need any help, just let me know.
Fixed for 7.5.20.