- A Spotlight importer, so Foxtrot can index.
- A Quicklook plugin, so we can view the preview in the app.
The same is true for EML/EMLX, or any other file type, by the way. I have checked Spotlight importers for EML/EMLX, since I tried Foxtrot a long time ago. I found, and recommended in the past, Eaglefiler, MailMate and Muttlight.
Quicklook plugins is another issue, because Apple's own Quicklook plugin used to work properly. Until Catalina. And for EMLX it doesn't work very well at all, although is manageable, at least for me.
The only solution for our EML/EMLX problem, is that someone develop a new plugin from scratch. There is a trick to make RTF Quicklook plugin to work with EML/EMLX, but is not the best solution.