fix(server): return 400 for unknown /upload/image type instead of 500

get_dir_by_type() only bound type_dir for dir_type in {None, "input",
"temp", "output"}. Any other value fell through to
`return type_dir, dir_type` with type_dir never assigned, raising
UnboundLocalError, which aiohttp surfaced as an unhandled HTTP 500.

POST /upload/image and POST /upload/mask both take the type value
straight from the multipart body, so a client typo such as type=foo
produced a server error where a client error is correct.

Return None for an unrecognised type and have image_upload answer 400.
Recognised values are unchanged.

server.py

@@ -377,6 +377,10 @@ class PromptServer():
                 type_dir = folder_paths.get_temp_directory()
             elif dir_type == "output":
                 type_dir = folder_paths.get_output_directory()
+            else:
+                # Unrecognised type: signal "no directory" instead of falling
+                # through to an unbound `type_dir` (UnboundLocalError -> 500).
+                return None, dir_type
 
             return type_dir, dir_type
 
@@ -401,6 +405,8 @@ class PromptServer():
 
             image_upload_type = post.get("type")
             upload_dir, image_upload_type = get_dir_by_type(image_upload_type)
+            if upload_dir is None:
+                return web.Response(status=400)
 
             if image and image.file:
                 filename = image.filename