MM-11868/MM-12010/MM-12011/MM-12036 Improve post metadata structure (#9693)

* MM-11868/MM-12010/MM-12011/MM-12036 Include dimensions for opengraph and message attachment images in post metadata

* Remove duplicate check from getFirstLinkAndImages

* Add tests for getImagesInMessageAttachments

* Rename PostMetadata.FileInfos to PostMetadata.Files

* Rename Metadata.ImageDimensions to Metadata.Images

model/post_embed.go

@@ -0,0 +1,22 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package model
+
+const (
+	POST_EMBED_IMAGE              PostEmbedType = "image"
+	POST_EMBED_MESSAGE_ATTACHMENT PostEmbedType = "message_attachment"
+	POST_EMBED_OPENGRAPH          PostEmbedType = "opengraph"
+)
+
+type PostEmbedType string
+
+type PostEmbed struct {
+	Type PostEmbedType `json:"type"`
+
+	// The URL of the embedded content. Used for image and OpenGraph embeds.
+	URL string `json:"url,omitempty"`
+
+	// Any additional data for the embedded content. Only used for OpenGraph embeds.
+	Data interface{} `json:"data,omitempty"`
+}