add file access field to file struct for slackevents (slack-go#1312)

Author: zFlabmonsta

slackevents/inner_events.go

@@ -472,6 +472,7 @@ type File struct {
 	DisplayAsBot       bool   `json:"display_as_bot"`
 	Username           string `json:"username"`
 	URLPrivate         string `json:"url_private"`
+	FileAccess         string `json:"file_access"`
 	URLPrivateDownload string `json:"url_private_download"`
 	Thumb64            string `json:"thumb_64"`
 	Thumb80            string `json:"thumb_80"`