This would be really easy to implement and have a big payback.
Grammarly is a very nice spelling and grammar checker that is free. (They offer a paid upgrade also) The free version is about the best “check as you type” auto correct I’ve seen.
Here is the problem: I tried to specify the Grammarly desktop app as my “external editor” in Joplin. Then tried to use it. Grammarly says it can only import a short list of file types “TXT” being one type. But as it turns out MD is actually text and Grammarly works well with Joplin stye MD files. But the app looks for the “TXT” extension.
Proposed fix. Allow some way to transform files names as they are exported. Maybe a symbolic link would work. When doing the export make a link “tempfile.txt --> Joplinfile.MD” and then give the external editor the temp filename.
Note that doing this also implements another much-requested feature – spell checking. Doing spell checking well is very hard. But implementing this filename redirection gives you what might be the best spell checker for a tiny amount of work.
In the meantime cut-and-past round trip joplin->grammarly-joplin does work. And Grammarly does the right thing and ignores the MD formatting characters.
I hate to advertise commercial software but if you want to look here is a link