[MM-14846] Update EditAt for FileIds and Attachment in Post + Ignore FileIds Updates (#10540)

* Set EditAt for FileIds and Attachments; Disallow update/patch of FileIds in API Handler

* Add custom comparison methods for StringArray and Post Attachments

* gofmt

* Split up comparison function to child structs

* Naming consistency

* gofmt

api4/post.go

@@ -437,6 +437,9 @@ func updatePost(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Updating the file_ids of a post is not a supported operation and will be ignored
+	post.FileIds = nil
+
 	if !c.App.SessionHasPermissionToChannelByPost(c.App.Session, c.Params.PostId, model.PERMISSION_EDIT_POST) {
 		c.SetPermissionError(model.PERMISSION_EDIT_POST)
 		return
@@ -479,6 +482,9 @@ func patchPost(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Updating the file_ids of a post is not a supported operation and will be ignored
+	post.FileIds = nil
+
 	if !c.App.SessionHasPermissionToChannelByPost(c.App.Session, c.Params.PostId, model.PERMISSION_EDIT_POST) {
 		c.SetPermissionError(model.PERMISSION_EDIT_POST)
 		return