diff --git a/poetry.lock b/poetry.lock
index 7ab990e76b..bf30fbbe15 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -1974,13 +1974,13 @@ six = ">=1.5"
 
 [[package]]
 name = "python-multipart"
-version = "0.0.10"
+version = "0.0.12"
 description = "A streaming multipart parser for Python"
 optional = false
 python-versions = ">=3.8"
 files = [
-    {file = "python_multipart-0.0.10-py3-none-any.whl", hash = "sha256:2b06ad9e8d50c7a8db80e3b56dab590137b323410605af2be20d62a5f1ba1dc8"},
-    {file = "python_multipart-0.0.10.tar.gz", hash = "sha256:46eb3c6ce6fdda5fb1a03c7e11d490e407c6930a2703fe7aef4da71c374688fa"},
+    {file = "python_multipart-0.0.12-py3-none-any.whl", hash = "sha256:43dcf96cf65888a9cd3423544dd0d75ac10f7aa0c3c28a175bbcd00c9ce1aebf"},
+    {file = "python_multipart-0.0.12.tar.gz", hash = "sha256:045e1f98d719c1ce085ed7f7e1ef9d8ccc8c02ba02b5566d5f7521410ced58cb"},
 ]
 
 [[package]]
diff --git a/synapse/http/client.py b/synapse/http/client.py
index 143fee9796..c3b2299c95 100644
--- a/synapse/http/client.py
+++ b/synapse/http/client.py
@@ -1039,7 +1039,7 @@ class _MultipartParserProtocol(protocol.Protocol):
         self.deferred = deferred
         self.boundary = boundary
         self.max_length = max_length
-        self.parser = None
+        self.parser: Optional[multipart.MultipartParser] = None
         self.multipart_response = MultipartResponse()
         self.has_redirect = False
         self.in_json = False
@@ -1097,7 +1097,7 @@ class _MultipartParserProtocol(protocol.Protocol):
                         self.deferred.errback()
                     self.file_length += end - start
 
-            callbacks = {
+            callbacks: "multipart.multipart.MultipartCallbacks" = {
                 "on_header_field": on_header_field,
                 "on_header_value": on_header_value,
                 "on_part_data": on_part_data,
@@ -1113,7 +1113,7 @@ class _MultipartParserProtocol(protocol.Protocol):
             self.transport.abortConnection()
 
         try:
-            self.parser.write(incoming_data)  # type: ignore[attr-defined]
+            self.parser.write(incoming_data)
         except Exception as e:
             logger.warning(f"Exception writing to multipart parser: {e}")
             self.deferred.errback()