Saving Emails from Thunderbird - That Open in Win7 Preview Pane Complete

You can Re-Save a Blocked Email, or Email that opens but does not display some Inline Images in the Windows 7 Preview Pane, using IE-11 as an (.mht) Web Page Complete File version.. and it "Will" open with the Inline Images Unblocked.

This seems to be 'Unique' to the file format IE-11 on Win7 saves it as, saving as an HTML file with Attachments or using some other Browsers Save As complete does not work.

I assume back in the Day, Microsoft made sure that IE-11 Save As would in some way specially prepare the html in .mht files that it alone generated, that would by-pass the Windows 7 Preview Pane filter controls.

I did try Unblocking using Windows Defender exceptions or turning that off, but it did not Unblock the Inline Images.

It might work all the way up to Win11 or using IE-11 to Re-Save or Preprocess since Microsoft tends to  not deprecate old code paths in new software

IE has of course been deprecated itself these days with the new Chrome based Bing browser. But you may be able to get IE-11 in a VM or some other place and use it simply as a tool for re-processing unviewable files.