Exclude sending file metadata to plugin hooks (#18454)

* Exclude sending file metadata to plugin hooks

A FileInfo object contained a MiniPreview which is
a slice of bytes. This can be particularly costly
while marshalling to plugin hooks.

We avoid this by refactoring the Embeds and Images
population to a separate method and calling that
to prevent posts from getting updated.

https://community-daily.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4?c=9c0b5413-5401-4ef2-83d5-b9f756585bbc

```release-note
NONE
```

* refactor to separate method

```release-note
NONE
```

app/post.go

@@ -297,7 +297,7 @@ func (a *App) CreatePost(c *request.Context, post *model.Post, channel *model.Ch
 		}
 	}
 
-	post = a.PreparePostForClient(post, true, false)
+	post = a.getEmbedsAndImages(post, true)
 	previewPost := post.GetPreviewPost()
 	if previewPost != nil {
 		post.AddProp(model.PostPropsPreviewedPost, previewPost.PostID)
@@ -521,7 +521,7 @@ func (a *App) SendEphemeralPost(userID string, post *model.Post) *model.Post {
 
 	post.GenerateActionIds()
 	message := model.NewWebSocketEvent(model.WebsocketEventEphemeralMessage, "", post.ChannelId, userID, nil)
-	post = a.PreparePostForClient(post, true, false)
+	post = a.PreparePostForClientWithEmbedsAndImages(post, true, false)
 	post = model.AddPostActionCookies(post, a.PostActionCookieSecret())
 
 	postJSON, jsonErr := post.ToJSON()
@@ -544,7 +544,7 @@ func (a *App) UpdateEphemeralPost(userID string, post *model.Post) *model.Post {
 
 	post.GenerateActionIds()
 	message := model.NewWebSocketEvent(model.WebsocketEventPostEdited, "", post.ChannelId, userID, nil)
-	post = a.PreparePostForClient(post, true, false)
+	post = a.PreparePostForClientWithEmbedsAndImages(post, true, false)
 	post = model.AddPostActionCookies(post, a.PostActionCookieSecret())
 	postJSON, jsonErr := post.ToJSON()
 	if jsonErr != nil {
@@ -685,7 +685,7 @@ func (a *App) UpdatePost(c *request.Context, post *model.Post, safeUpdate bool)
 		})
 	}
 
-	rpost = a.PreparePostForClient(rpost, false, true)
+	rpost = a.PreparePostForClientWithEmbedsAndImages(rpost, false, true)
 
 	// Ensure IsFollowing is nil since this updated post will be broadcast to all users
 	// and we don't want to have to populate it for every single user and broadcast to each