[tmp] Provide MSC2530 event content

Required until mautrix/python#170 is merged and
pulled into maubot.

Signed-off-by: Joe Groocock <jgroocock@cloudflare.com>

Author: frebib

xkcd.py

@@ -42,6 +42,13 @@
     Image = None
 
 
+@dataclass
+class MSC2530MediaMessageEventContent(MediaMessageEventContent):
+    filename: str = None
+    formatted_body: str = None
+    format: Format = None
+
+
 @dataclass
 class XKCDInfo(SerializableAttrs):
     year: str
@@ -246,7 +253,7 @@ async def _send_xkcd(self, room_id: RoomID, xkcd: XKCDInfo) -> None:
             content["license_url"] = "https://xkcd.com/license.html"
             await self.client.send_message(room_id, content)
         else:
-            content = MediaMessageEventContent(
+            content = MSC2530MediaMessageEventContent(
                 msgtype=MessageType.IMAGE,
                 format=Format.HTML,
                 external_url=f"https://xkcd.com/{xkcd.num}",