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_metadata.go

@@ -60,7 +60,7 @@ func (a *App) PreparePostListForClient(originalList *model.PostList) *model.Post
 	}
 
 	for id, originalPost := range originalList.Posts {
-		post := a.PreparePostForClient(originalPost, false, false)
+		post := a.PreparePostForClientWithEmbedsAndImages(originalPost, false, false)
 
 		list.Posts[id] = post
 	}
@@ -94,15 +94,16 @@ func (a *App) OverrideIconURLIfEmoji(post *model.Post) {
 	}
 }
 
-func (a *App) PreparePostForClient(originalPost *model.Post, isNewPost bool, isEditPost bool) *model.Post {
+func (a *App) PreparePostForClient(originalPost *model.Post, isNewPost, isEditPost bool) *model.Post {
 	post := originalPost.Clone()
 
 	// Proxy image links before constructing metadata so that requests go through the proxy
 	post = a.PostWithProxyAddedToImageURLs(post)
 
 	a.OverrideIconURLIfEmoji(post)
-
-	post.Metadata = &model.PostMetadata{}
+	if post.Metadata == nil {
+		post.Metadata = &model.PostMetadata{}
+	}
 
 	if post.DeleteAt > 0 {
 		// For deleted posts we don't fill out metadata nor do we return the post content
@@ -125,9 +126,22 @@ func (a *App) PreparePostForClient(originalPost *model.Post, isNewPost bool, isE
 		post.Metadata.Files = fileInfos
 	}
 
+	return post
+}
+
+func (a *App) PreparePostForClientWithEmbedsAndImages(originalPost *model.Post, isNewPost, isEditPost bool) *model.Post {
+	post := a.PreparePostForClient(originalPost, isNewPost, isEditPost)
+	post = a.getEmbedsAndImages(post, true)
+	return post
+}
+
+func (a *App) getEmbedsAndImages(post *model.Post, isNewPost bool) *model.Post {
+	if post.Metadata == nil {
+		post.Metadata = &model.PostMetadata{}
+	}
+
 	// Embeds and image dimensions
 	firstLink, images := a.getFirstLinkAndImages(post.Message)
-
 	if embed, err := a.getEmbedForPost(post, firstLink, isNewPost); err != nil {
 		appErr, ok := err.(*model.AppError)
 		isNotFound := ok && appErr.StatusCode == http.StatusNotFound
@@ -142,7 +156,6 @@ func (a *App) PreparePostForClient(originalPost *model.Post, isNewPost bool, isE
 	}
 
 	post.Metadata.Images = a.getImagesForPost(post, images, isNewPost)
-
 	return post
 }