Request song covers through MTProto.

Telegram/SourceFiles/data/data_document.h

@@ -285,6 +285,7 @@ private:
 		InlineThumbnailIsPath = 0x080,
 		ForceToCache = 0x100,
 		PremiumSticker = 0x200,
+		PossibleCoverThumbnail = 0x400,
 	};
 	using Flags = base::flags<Flag>;
 	friend constexpr bool is_flag_type(Flag) { return true; };
@@ -326,6 +327,8 @@ private:
 
 	bool saveFromDataChecked();
 
+	void refreshPossibleCoverThumbnail();
+
 	const not_null<Data::Session*> _owner;
 
 	// Two types of location: from MTProto by dc+access or from web by url