[MM-46097] Change FileInfo count query (#20871)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/sqlstore/channel_store.go

@@ -2216,11 +2216,10 @@ func (s SqlChannelStore) GetFileCount(channelId string) (int64, error) {
 		    COUNT(*)
 		FROM
 		    FileInfo
-                LEFT JOIN Posts as P ON FileInfo.PostId=P.Id
-                LEFT JOIN Channels as C on C.Id=P.ChannelId
 		WHERE
 		    FileInfo.DeleteAt = 0
-			AND C.Id = ?`, channelId)
+            AND FileInfo.PostId IN (SELECT id FROM Posts WHERE ChannelId = ?)`,
+		channelId)
 	if err != nil {
 		return 0, errors.Wrapf(err, "failed to count files with channelId=%s", channelId)
 	}