Determine the content type of a file based on charset and mime type.
@param  {Object} file
@return {String}

@api private
function getContentType(file) {
  var mimeType =
    mime.lookup(file.unzipPath || file.path) || 'application/octet-stream';
  var charset = mime.charset(mimeType);

  return charset ? mimeType + '; charset=' + charset.toLowerCase() : mimeType;
}