docs: add Http.File response body section

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

Author: msyyc

docs/developer/http_file_body_compatibility.md

@@ -1,10 +1,17 @@
-# `Http.File` as Python Request Body
+# `Http.File` Compatibility for Python
 
 ## Overview
 
-TypeSpec `Http.File` maps to `IO | bytes` in generated Python SDK methods. This widens the Swagger `type: file` signature (previously `bytes` only) while keeping full backward compatibility.
+TypeSpec `Http.File` is emitted differently depending on whether it appears as a request body or a response body:
 
-## TypeSpec Definition
+- **Request body**: emits `IO | bytes` (widens legacy `bytes`-only signature)
+- **Response body**: emits `bytes` (identical to legacy signature)
+
+Both cases preserve full backward compatibility with Swagger `type: file`.
+
+## Request Body
+
+### TypeSpec Definition
 
 ```typespec
 import "@typespec/http";
@@ -18,7 +25,7 @@ namespace Files;
 op upload(@body body: Http.File): void;
 ```
 
-## Generated Python SDK
+### Generated Python SDK
 
 ```python
 from typing import IO, overload
@@ -35,7 +42,30 @@ def upload(self, body: IO | bytes, **kwargs) -> None:
     ...
 ```
 
-## Backward Compatibility
+### Backward Compatibility
 
 Swagger `type: file`(e.g. [`operationId: "WebApps_GetWebSiteContainerLogs"`](https://github.com/Azure/azure-rest-api-specs/blob/main/specification/web/resource-manager/Microsoft.Web/AppService/stable/2024-11-01/WebApps.json#L2675)) generated `bytes`-only input (e.g. [`WebAppsOperations.get_web_site_container_logs(...)`](https://github.com/Azure/azure-sdk-for-python/blob/main/sdk/appservice/azure-mgmt-web/azure/mgmt/web/operations/_web_apps_operations.py#L22070)). After migrating to TypeSpec, the input type widens to `IO | bytes`. Since `bytes` is still accepted, existing callers require **no code changes**, making this migration **non-breaking**.
 
+## Response Body
+
+### TypeSpec Definition
+
+```typespec
+@route("/files")
+namespace Files;
+
+@get
+op download(): Http.File;
+```
+
+### Generated Python SDK
+
+```python
+def download(self, **kwargs) -> bytes:
+    ...
+```
+
+### Backward Compatibility
+
+Swagger `type: file` responses also generated `bytes`. The TypeSpec `Http.File` response emits the same `bytes` return type, so existing callers are **completely unaffected**.
+

docs/developer/readme.md

@@ -15,4 +15,4 @@ To test an unpublished branded emitter tarball, please follow these steps:
 
 ## Design Notes
 
-- [`Http.File` request body compatibility for Python](./http_file_body_compatibility.md)
+- [`Http.File` compatibility for Python](./http_file_body_compatibility.md)