I don't know how to navigate to a mmap file inside a channel and open the file. Teams does not yet recognize the mmap extension and seems to offer only the option to download the file. Any solutions or workarounds??
You'll need to install MindManager for Microsoft Teams in order to open files inside Teams. The video posted by Joerg goes over the details of how to set up a MindManager tab that can display and allow opening of files inside Teams.
Currently MindManager for Microsoft Teams is available for enterprises to download from our website, it requires a Microsoft 365 admin to install. The app will be available in the Microsoft App Source (store) inside Teams later this year. After that, individual Teams users will be able to install the application without the aid of their Microsoft 365 admin as long as their organization's policy allows installation of 3rd party apps.