I'm setting up my computer and I have most everything I want set up, but the problem I'm running into is my email is handled by Tutanota, which is proprietary and cannot be added to generic email clients. There is a desktop client, but it's an appimage and I can't for the life of me figure out how to set it as default. When I access webmail, it immediate prompts if I'd like to use that as my default mail, which I will do if I can't get this working. I found this thread:

How can i set Lotus Notes as the default mail program?

And I tried to apply some of the information there to my problem, but I suspect the fact that I have an appimage and not an actual installed application might be an unaccounted for confounding factor. When I try to run gedit on the appimage it takes several minutes for text edit to open and displays a lot of unloaded characters and red double zeroes and I just got really intimidated, to be quite honest. I tried to follow the suggestion of someone a bit further down to edit mimeapps.list which is not present in that directory on my system, but I did find mimeinfo.cache which included lines like the one suggested, so I edited that as directed, but to no avail.

I also found this thread:

Unable to choose default application

And while I was able to use the information there to enable me to open up saved raw emails with my desktop client, I'm still unable to add it as my default mail app.

I feel like I'm missing something. Does anyone have any ideas on how to use the Tutanota desktop client appimage as the default mail app in Ubuntu?

UPDATE: I tested with a couple of different versions of Thunderbird because I know that's the default mail client for Ubuntu. As expected, the appimage version did not display as an option for a default email client, but the manual download from the website didn't either, and I was able to open and use that app as expected. The only version of Thunderbird that the system would actually recognize as a mail client in settings was the one downloaded from Ubuntu Software. I am still unsure what the difference is, so if anyone knows please tell me. I think I may just have to stick with webmail as my default "email client" if it will only recognize apps from Ubuntu Software.