fix: use FileUrl.format to find the extension (#2280)

Author: Kludex

pydantic_ai_slim/pydantic_ai/models/__init__.py

@@ -758,9 +758,7 @@ async def download_item(
 
     data_type = media_type
     if type_format == 'extension':
-        data_type = data_type.split('/')[1]
-        if data_type == 'mpeg':  # pragma: no cover
-            data_type = 'mp3'
+        data_type = item.format
 
     data = response.content
     if data_format in ('base64', 'base64_uri'):