diff --git a/api/core/workflow/nodes/http_request/executor.py b/api/core/workflow/nodes/http_request/executor.py
index 2c42f5a1be..34a6dd8175 100644
--- a/api/core/workflow/nodes/http_request/executor.py
+++ b/api/core/workflow/nodes/http_request/executor.py
@@ -235,6 +235,10 @@ class Executor:
                                 files[key].append(file_tuple)
 
                     # convert files to list for httpx request
+                    # If there are no actual files, we still need to force httpx to use `multipart/form-data`.
+                    # This is achieved by inserting a harmless placeholder file that will be ignored by the server.
+                    if not files:
+                        self.files = [("__multipart_placeholder__", ("", b"", "application/octet-stream"))]
                     if files:
                         self.files = []
                         for key, file_tuples in files.items():