This gist maps all the file extensions to supported monaco editor languages. This is based on monaco docs and github's language to extension map.

file-extension-to-language.json

{
  "_coffee": "coffeescript",
  "_js": "javascript",
  "adp": "tcl",
  "al": "perl",
  "ant": "xml",
  "aw": "php",
  "axml": "xml",
  "bash": "shell",
  "bats": "shell",
  "bones": "javascript",
  "boot": "clojure",
  "builder": "ruby",
  "bzl": "python",
  "c": "c",
  "c++": "cpp",
  "cake": "coffeescript",
  "cats": "c",
  "cc": "cpp",
  "ccxml": "xml",
  "cfg": "ini",
  "cgi": "shell",
  "cjsx": "coffeescript",
  "cl2": "clojure",
  "clixml": "xml",
  "clj": "clojure",
  "cljc": "clojure",
  "cljs.hl": "clojure",
  "cljs": "clojure",
  "cljscm": "clojure",
  "cljx": "clojure",
  "coffee": "coffeescript",
  "command": "shell",
  "cp": "cpp",
  "cpp": "cpp",
  "cproject": "xml",
  "cql": "sql",
  "csl": "xml",
  "cson": "coffeescript",
  "csproj": "xml",
  "ct": "xml",
  "ctp": "php",
  "cxx": "cpp",
  "ddl": "sql",
  "dfm": "pascal",
  "dita": "xml",
  "ditamap": "xml",
  "ditaval": "xml",
  "dll.config": "xml",
  "dotsettings": "xml",
  "dpr": "pascal",
  "ecl": "ecl",
  "eclxml": "ecl",
  "es": "javascript",
  "es6": "javascript",
  "ex": "elixir",
  "exs": "elixir",
  "fcgi": "shell",
  "filters": "xml",
  "frag": "javascript",
  "fsproj": "xml",
  "fxml": "xml",
  "gemspec": "ruby",
  "geojson": "json",
  "glade": "xml",
  "gml": "xml",
  "god": "ruby",
  "grxml": "xml",
  "gs": "javascript",
  "gyp": "python",
  "h": "cpp",
  "h++": "cpp",
  "handlebars": "handlebars",
  "hbs": "handlebars",
  "hcl": "hcl",
  "hh": "cpp",
  "hic": "clojure",
  "hpp": "cpp",
  "htm": "html",
  "html.hl": "html",
  "html": "html",
  "hxx": "cpp",
  "iced": "coffeescript",
  "idc": "c",
  "iml": "xml",
  "inc": "sql",
  "ini": "ini",
  "inl": "cpp",
  "ipp": "cpp",
  "irbrc": "ruby",
  "ivy": "xml",
  "j2": "python",
  "jake": "javascript",
  "jbuilder": "ruby",
  "jelly": "xml",
  "jinja": "python",
  "jinja2": "python",
  "js": "javascript",
  "jsb": "javascript",
  "jscad": "javascript",
  "jsfl": "javascript",
  "jsm": "javascript",
  "json": "json",
  "jsproj": "xml",
  "jss": "javascript",
  "kml": "xml",
  "ksh": "shell",
  "kt": "kotlin",
  "ktm": "kotlin",
  "kts": "kotlin",
  "launch": "xml",
  "lmi": "python",
  "lock": "json",
  "lpr": "pascal",
  "lua": "lua",
  "markdown": "markdown",
  "md": "markdown",
  "mdpolicy": "xml",
  "mkd": "markdown",
  "mkdn": "markdown",
  "mkdown": "markdown",
  "mm": "xml",
  "mod": "xml",
  "mspec": "ruby",
  "mustache": "python",
  "mxml": "xml",
  "njs": "javascript",
  "nproj": "xml",
  "nse": "lua",
  "nuspec": "xml",
  "odd": "xml",
  "osm": "xml",
  "pac": "javascript",
  "pas": "pascal",
  "pd_lua": "lua",
  "perl": "perl",
  "ph": "perl",
  "php": "php",
  "php3": "php",
  "php4": "php",
  "php5": "php",
  "phps": "php",
  "phpt": "php",
  "pl": "perl",
  "plist": "xml",
  "pluginspec": "xml",
  "plx": "perl",
  "pm": "perl",
  "pod": "perl",
  "podspec": "ruby",
  "pp": "pascal",
  "prc": "sql",
  "prefs": "ini",
  "pro": "ini",
  "properties": "ini",
  "props": "xml",
  "ps1": "powershell",
  "ps1xml": "xml",
  "psc1": "xml",
  "psd1": "powershell",
  "psgi": "perl",
  "psm1": "powershell",
  "pt": "xml",
  "py": "python",
  "pyde": "python",
  "pyp": "python",
  "pyt": "python",
  "pyw": "python",
  "r": "r",
  "rabl": "ruby",
  "rake": "ruby",
  "rb": "ruby",
  "rbuild": "ruby",
  "rbw": "ruby",
  "rbx": "ruby",
  "rbxs": "lua",
  "rd": "r",
  "rdf": "xml",
  "reek": "yaml",
  "rest.txt": "restructuredtext",
  "rest": "restructuredtext",
  "ron": "markdown",
  "rpy": "python",
  "rq": "sparql",
  "rs.in": "rust",
  "rs": "rust",
  "rss": "xml",
  "rst.txt": "restructuredtext",
  "rst": "restructuredtext",
  "rsx": "r",
  "ru": "ruby",
  "ruby": "ruby",
  "rviz": "yaml",
  "sbt": "scala",
  "sc": "scala",
  "scala": "scala",
  "scm": "scheme",
  "scxml": "xml",
  "sh.in": "shell",
  "sh": "shell",
  "sjs": "javascript",
  "sld": "scheme",
  "sls": "scheme",
  "sparql": "sparql",
  "sps": "scheme",
  "sql": "sql",
  "srdf": "xml",
  "ss": "scheme",
  "ssjs": "javascript",
  "st": "html",
  "storyboard": "xml",
  "sttheme": "xml",
  "sublime_metrics": "javascript",
  "sublime_session": "javascript",
  "sublime-build": "javascript",
  "sublime-commands": "javascript",
  "sublime-completions": "javascript",
  "sublime-keymap": "javascript",
  "sublime-macro": "javascript",
  "sublime-menu": "javascript",
  "sublime-mousemap": "javascript",
  "sublime-project": "javascript",
  "sublime-settings": "javascript",
  "sublime-snippet": "xml",
  "sublime-syntax": "yaml",
  "sublime-theme": "javascript",
  "sublime-workspace": "javascript",
  "sv": "systemverilog",
  "svh": "systemverilog",
  "syntax": "yaml",
  "t": "perl",
  "tab": "sql",
  "tac": "python",
  "targets": "xml",
  "tcc": "cpp",
  "tcl": "tcl",
  "tf": "hcl",
  "thor": "ruby",
  "tm": "tcl",
  "tmcommand": "xml",
  "tml": "xml",
  "tmlanguage": "xml",
  "tmpreferences": "xml",
  "tmsnippet": "xml",
  "tmtheme": "xml",
  "tmux": "shell",
  "tool": "shell",
  "topojson": "json",
  "tpp": "cpp",
  "ts": "typescript",
  "tsx": "typescript",
  "udf": "sql",
  "ui": "xml",
  "urdf": "xml",
  "ux": "xml",
  "v": "verilog",
  "vbproj": "xml",
  "vcxproj": "xml",
  "veo": "verilog",
  "vh": "systemverilog",
  "viw": "sql",
  "vssettings": "xml",
  "vxml": "xml",
  "w": "c",
  "watchr": "ruby",
  "wlua": "lua",
  "wsdl": "xml",
  "wsf": "xml",
  "wsgi": "python",
  "wxi": "xml",
  "wxl": "xml",
  "wxs": "xml",
  "x3d": "xml",
  "xacro": "xml",
  "xaml": "xml",
  "xht": "html",
  "xhtml": "html",
  "xib": "xml",
  "xlf": "xml",
  "xliff": "xml",
  "xmi": "xml",
  "xml.dist": "xml",
  "xml": "xml",
  "xproj": "xml",
  "xpy": "python",
  "xsd": "xml",
  "xsjs": "javascript",
  "xsjslib": "javascript",
  "xul": "xml",
  "yaml-tmlanguage": "yaml",
  "yaml": "yaml",
  "yml": "yaml",
  "zcml": "xml",
  "zsh": "shell"
}

11/10 overkill