code: func (F *Frisby) AddFile(filename string) *Frisby {
  	file, err := os.Open(filename)
  	if err != nil {
  		F.Errs = append(F.Errs, err)
  	} else {
  		fileField := request.FileField{
  			FieldName: defaultFileKey,
  			FileName:  filepath.Base(filename),
  			File:      file}
  		F.Req.Files = append(F.Req.Files, fileField)
  	}
  	return F
  }
docs: // Add a file to the Form data for the coming request