I have had a simiar issue before.
Basically, the developer of the app has to make the app able to be seen as able to open such a file.

There is an app that I have on my devices that seems to have helped.
As it is not intrusive once you set it up, it is pretty hard to even remember that it is there.

Maybe it can help you out.

https://f-droid.org/en/packages/com.tasomaniac.openwith.floss/

If this is of no help, then all I can say is to download the file and then open it with the app of your choice.

If this is not possible, download the file and then find it with your file manager.
Long press it, and then select 'share'.
Hopefully the app you desire will show up as an option to share it to.

Maybe that is the solution after all, to share the file with the app, rather than trying to open it with he app?

 

This is the key thing to understand. The operating system does not analyse apps and work out which ones can open a particular type of file. Rather the app declares which type of file it can open (in the technical language of android which "intents" it can handle, which is slightly broader than just opening files) and the OS offers you a list of apps that have said they can do this. So as said above, if the app developer does not declare that their app can do this the operating system has no reason to offer it as an option.

You could try contacting the developer and seeing whether they are willing to fix this.

As for the apps that do offer but can't open it, that may be because their developers made an error and declared that they could do things they can't, or it may be because the relevant intent is more broadly defined and includes other things that they can handle but not the particular markdown files you are interested in - I'm afraid I can't say on this.

 

Just use the app that opens the md file.
Don't use your file manager app