Use MediaType returned by ImageGenerator (#50079)

Author: ericstj

docs/ai/quickstarts/snippets/text-to-image/hosting/Program.cs

@@ -37,7 +37,7 @@
     ImageGenerationResponse response = await generator.GenerateImagesAsync(prompt);
     DataContent dataContent = response.Contents.OfType<DataContent>().First();
 
-    return Results.File(dataContent.Data.ToArray(), "image/png");
+    return Results.File(dataContent.Data.ToArray(), dataContent.MediaType);
 });
 // </SnippetUseImageGenerator>