Fix test util typings (#437)

jupyterlab_server/test_utils.py

@@ -60,6 +60,10 @@ def __init__(self, request: HTTPRequest, spec: Spec):
             path=path,
         )
 
+    @property
+    def content_type(self) -> str:
+        return "application/json"
+
     @property
     def host_url(self) -> str:
         url = self.request.url
@@ -95,13 +99,13 @@ def method(self) -> str:
         return method and method.lower() or ""
 
     @property
-    def body(self) -> str | None:
+    def body(self) -> bytes | None:
         if self.request.body is None:
             return None  # type:ignore[unreachable]
         if not isinstance(self.request.body, bytes):
             msg = "Request body is invalid"  # type:ignore[unreachable]
             raise AssertionError(msg)
-        return self.request.body.decode("utf-8")
+        return self.request.body
 
     @property
     def mimetype(self) -> str:
@@ -123,16 +127,20 @@ def __init__(self, response: HTTPResponse):
         self.response = response
 
     @property
-    def data(self) -> str:
+    def data(self) -> bytes | None:
         if not isinstance(self.response.body, bytes):
             msg = "Response body is invalid"  # type:ignore[unreachable]
             raise AssertionError(msg)
-        return self.response.body.decode("utf-8")
+        return self.response.body
 
     @property
     def status_code(self) -> int:
         return int(self.response.code)
 
+    @property
+    def content_type(self) -> str:
+        return "application/json"
+
     @property
     def mimetype(self) -> str:
         return str(self.response.headers.get("Content-Type", "application/json"))