diff --git a/starlette/datastructures.py b/starlette/datastructures.py
index 7aae90ec..f49dbc35 100644
--- a/starlette/datastructures.py
+++ b/starlette/datastructures.py
@@ -477,7 +477,7 @@ class UploadFile:
             await run_in_threadpool(self.file.close)
 
 
-class FormData(ImmutableMultiDict):
+class FormData(ImmutableMultiDict[str, typing.Union[UploadFile, str]]):
     """
     An immutable multidict, containing both file uploads and text input.
     """
diff --git a/tests/test_formparsers.py b/tests/test_formparsers.py
index 7418595c..b7f8cad8 100644
--- a/tests/test_formparsers.py
+++ b/tests/test_formparsers.py
@@ -24,7 +24,7 @@ FORCE_MULTIPART = ForceMultipartDict()
 async def app(scope, receive, send):
     request = Request(scope, receive)
     data = await request.form()
-    output = {}
+    output: typing.Dict[str, typing.Any] = {}
     for key, value in data.items():
         if isinstance(value, UploadFile):
             content = await value.read()
@@ -66,7 +66,7 @@ async def multi_items_app(scope, receive, send):
 async def app_with_headers(scope, receive, send):
     request = Request(scope, receive)
     data = await request.form()
-    output = {}
+    output: typing.Dict[str, typing.Any] = {}
     for key, value in data.items():
         if isinstance(value, UploadFile):
             content = await value.read()