Add changeset for multipart filename fix

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: iscai-msft

.chronus/changes/fix-multipart-filename-2026-5-29-17-50-0.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-client-python"
+---
+
+Synthesize filename in multipart Content-Disposition for bare file inputs. When callers pass bare bytes/str/IO instead of a (filename, content) tuple for multipart file fields, the `prepare_multipart_form_data` helper now wraps them with a synthesized filename so servers that require `filename=` in the Content-Disposition header no longer reject the upload.