diff --git a/changelog.d/15137.removal b/changelog.d/15137.removal
new file mode 100644
index 0000000000..c533b0c9dd
--- /dev/null
+++ b/changelog.d/15137.removal
@@ -0,0 +1 @@
+Remove the undocumented and unspecced `type` parameter to the `/thumbnail` endpoint.
diff --git a/synapse/rest/media/v1/thumbnail_resource.py b/synapse/rest/media/v1/thumbnail_resource.py
index 5f725c7600..3e720018b3 100644
--- a/synapse/rest/media/v1/thumbnail_resource.py
+++ b/synapse/rest/media/v1/thumbnail_resource.py
@@ -69,7 +69,8 @@ class ThumbnailResource(DirectServeJsonResource):
         width = parse_integer(request, "width", required=True)
         height = parse_integer(request, "height", required=True)
         method = parse_string(request, "method", "scale")
-        m_type = parse_string(request, "type", "image/png")
+        # TODO Parse the Accept header to get an prioritised list of thumbnail types.
+        m_type = "image/png"
 
         if server_name == self.server_name:
             if self.dynamic_thumbnails: