code: function populateMaps (extensions, types) {
    // source preference (least -> most)
    var preference = ['nginx', 'apache', undefined, 'iana']
  
    Object.keys(db).forEach(function forEachMimeType (type) {
      var mime = db[type]
      var exts = mime.extensions
  
      if (!exts || !exts.length) {
        return
      }
  
      // mime -> extensions
      extensions[type] = exts
  
      // extension -> mime
      for (var i = 0; i < exts.length; i++) {
        var extension = exts[i]
  
        if (types[extension]) {
          var from = preference.indexOf(db[types[extension]].source)
          var to = preference.indexOf(mime.source)
  
          if (types[extension] !== 'application/octet-stream' &&
            (from > to || (from === to && types[extension].substr(0, 12) === 'application/'))) {
            // skip the remapping
            continue
          }
        }
  
        // set the extension -> mime
        types[extension] = type
      }
    })
  }
docs: Populate the extensions and types maps.
  @private