(Allowed) Remote content not downloaded/cached for offline use

I use IMAP and I've allowed remote content from particular sources (not all). Sometimes I work in offline mode. When I do, I accept Thunderbird's offer to "download messages for offline use before going offline." This appears to download my IMAP server's messages, but not the (allowed) remote content. As a workaround, I've found that opening each message in my inbox before going offline is sufficient to cache the remote content, but that's tedious. It'd be nice if the "download messages for offline use" would cache the (allowed) remote content when it downloads the messages.

So I think this is a feature request unless I'm doing something wrong. And if this is the wrong place to submit feature requests, please let me know. Thank you!