code: func (v *Validator) AcceptFileExts(field string, exts ...string) *ValidationResult {
  	if !v.data.FileExists(field) {
  		return validationOk
  	}
  	header := v.data.GetFile(field)
  	gotExt := filepath.Ext(header.Filename)
  	for _, ext := range exts {
  		if ext == gotExt[1:] {
  			return validationOk
  		}
  	}
  	return v.addFileExtError(field, gotExt, exts...)
  }
docs: // AcceptFileExts will add an error to the Validator if the extension
  // of the file identified by field is not in exts. exts should be one ore more
  // allowed file extensions, not including the preceding ".". If the file does not
  // exist, it does not add an error to the Validator.