code: function contentType (str) {
    // TODO: should this even be in this module?
    if (!str || typeof str !== 'string') {
      return false
    }
  
    var mime = str.indexOf('/') === -1
      ? exports.lookup(str)
      : str
  
    if (!mime) {
      return false
    }
  
    // TODO: use content-type or other module
    if (mime.indexOf('charset') === -1) {
      var charset = exports.charset(mime)
      if (charset) mime += '; charset=' + charset.toLowerCase()
    }
  
    return mime
  }
docs: Create a full Content-Type header given a MIME type or extension.
  
  @param {string} str
  @return {boolean|string}