code: async function validateFile (file, options) {
    if (file instanceof File === false) {
      return null
    }
  
    await file.setOptions(Object.assign(file.validationOptions, options)).runValidations()
    return _.size(file.error()) ? file.error().message : null
  }
docs: Validates a single file instance. If validation has error, then error
  message will be returned as string, else null is returned.
  
  @method validateFile
  
  @param  {File}           file
  @param  {Object}           options
  
  @return {String|Null}