diff --git a/scrapy/responsetypes.py b/scrapy/responsetypes.py
index cd62f02af..29e9b6bb7 100644
--- a/scrapy/responsetypes.py
+++ b/scrapy/responsetypes.py
@@ -40,18 +40,13 @@ class ResponseTypes:
         self.classes: dict[str, type[Response]] = {}
         self.mimetypes: MimeTypes = MimeTypes()
         mimedata = get_data("scrapy", "mime.types")
-        if not mimedata:
-            raise ValueError(
-                "The mime.types file is not found in the Scrapy installation"
-            )
+        assert mimedata is not None
         self.mimetypes.readfp(StringIO(mimedata.decode("utf8")))
         for mimetype, cls in self.CLASSES.items():
             self.classes[mimetype] = load_object(cls)
 
     def from_mimetype(self, mimetype: str) -> type[Response]:
         """Return the most appropriate Response class for the given mimetype"""
-        if mimetype is None:
-            return Response
         if mimetype in self.classes:
             return self.classes[mimetype]
         basetype = f"{mimetype.split('/', maxsplit=1)[0]}/*"
diff --git a/tests/test_responsetypes.py b/tests/test_responsetypes.py
index 5b04c7436..42ab29267 100644
--- a/tests/test_responsetypes.py
+++ b/tests/test_responsetypes.py
@@ -40,6 +40,9 @@ class TestResponseTypes:
             retcls = responsetypes.from_content_disposition(source)
             assert retcls is cls, f"{source} ==> {retcls} != {cls}"
 
+    def test_from_content_disposition_no_filename(self):
+        assert responsetypes.from_content_disposition(b"attachment") is Response
+
     def test_from_content_type(self):
         mappings = [
             ("text/html; charset=UTF-8", HtmlResponse),