/node_modules/mime/build/build.js

 1var db = require('mime-db');
 2
 3var mapByType = {};
 4Object.keys(db).forEach(function(key) {
 5  var extensions = db[key].extensions;
 6  if (extensions) {
 7    mapByType[key] = extensions;
 8  }
 9});
10
11console.log(JSON.stringify(mapByType));