{
    modes: {
        html: {
            name: "HTML",
            highlighter: "ace/mode/html",
            extensions: [
                "html"
            ],
            commands: {
                "Tools:Beautify": {
                    scriptUrl: "/default/mode/html/beautify.js",
                    inputs: {
                        preferences: true
                    }
                },
                "Tools:Preview": {
                    scriptUrl: "/default/mode/html/preview.js"
                }
            },
            handlers: {
                preview: [
                    "Tools:Preview"
                ]
            }
        }
    }
}

{
    modes: {
        html: {
            name: "HTML",
            highlighter: "ace/mode/html",
            extensions: [
                "html",
                "htm"
            ],
            commands: {
                "Tools:Beautify": {
                    scriptUrl: "/default/mode/html/beautify.js",
                    inputs: {
                        preferences: true
                    }
                },
                "Tools:Preview": {
                    scriptUrl: "/default/mode/html/preview.js"
                }
            },
            handlers: {
                preview: [
                    "Tools:Preview"
                ]
            }
        }
    }
}

Add .htm file extension to HTML mode
Add .htm file extension to HTML mode
