torbirdy/thunderbird opens html-attachment in thunderbird and loads content from the internet
I tried to send a news-message from an rss-feed by e-mail, so I put the rss-feed-message into the attachment area and than clicked on the attachment (mail wasn't send yet) (the rss-feed is displayed as plain text) but if I click on the the attachment it's opened as html like in a browser (but in Thunderbird) and even the images are loaded and displayed.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information