Computer Configuration > Administrative Templates > Microsoft Edge
Both
HKLM\Software\Policies\Microsoft\Edge
This policy controls whether local mht or mhtml files launched from the command line can open automatically in Internet Explorer mode based on the file content without specifying the --ie-mode-file-url command line. This setting works in conjunction with: 'InternetExplorerIntegrationLevel' (Configure Internet Explorer integration) is set to 'IEMode' and 'InternetExplorerIntegrationLocalFileAllowed' (Allow launching of local files in Internet Explorer mode) is enabled or not configured. If you enable or don't configure this policy, local mht or mhtml files can launch in Microsoft Edge or Internet Explorer mode to best view the file. If you disable this policy, local mht or mhtml files will launch in Microsoft Edge. Note that if you use the --ie-mode-file-url command line argument for launching local mht or mhtml files, it takes precedence over how you configured this policy. To learn more about Internet Explorer mode, see https://go.microsoft.com/fwlink/?linkid=2094210