diff --git a/chatkit/server.py b/chatkit/server.py
index 01372cb..65645f6 100644
--- a/chatkit/server.py
+++ b/chatkit/server.py
@@ -435,6 +435,7 @@ async def _process_non_streaming(
                 attachment = await attachment_store.create_attachment(
                     request.params, context
                 )
+                await self.store.save_attachment(attachment, context=context)
                 return self._serialize(attachment)
             case AttachmentsDeleteReq():
                 attachment_store = self._get_attachment_store()
diff --git a/chatkit/types.py b/chatkit/types.py
index a0ceeea..cbb5900 100644
--- a/chatkit/types.py
+++ b/chatkit/types.py
@@ -726,16 +726,27 @@ class ToolChoice(BaseModel):
     id: str
 
 
+class AttachmentUploadDescriptor(BaseModel):
+    """Two-phase upload instructions."""
+
+    url: AnyUrl
+    method: Literal["POST", "PUT"]
+    """The HTTP method to use when uploading the file for two-phase upload."""
+    headers: dict[str, str] = Field(default_factory=dict)
+    """Optional headers to include in the upload request."""
+
+
 class AttachmentBase(BaseModel):
     """Base metadata shared by all attachments."""
 
     id: str
     name: str
     mime_type: str
-    upload_url: AnyUrl | None = None
+    upload_descriptor: AttachmentUploadDescriptor | None = None
     """
-    The URL to upload the file, used for two-phase upload.
-    Should be set to None after upload is complete or when using direct upload where uploading happens when creating the attachment object.
+    Two-phase upload instructions.
+    Should be set to None after upload is complete or when using direct upload
+    where uploading happens when creating the attachment object.
     """
 
 
diff --git a/tests/test_chatkit_server.py b/tests/test_chatkit_server.py
index 41be193..ff7177c 100644
--- a/tests/test_chatkit_server.py
+++ b/tests/test_chatkit_server.py
@@ -31,6 +31,7 @@
     AttachmentDeleteParams,
     AttachmentsCreateReq,
     AttachmentsDeleteReq,
+    AttachmentUploadDescriptor,
     ClientToolCallItem,
     FeedbackKind,
     FileAttachment,
@@ -107,14 +108,22 @@ async def create_attachment(
                 mime_type=input.mime_type,
                 name=input.name,
                 preview_url=AnyUrl(f"https://example.com/{id}/preview"),
-                upload_url=AnyUrl(f"https://example.com/{id}/upload"),
+                upload_descriptor=AttachmentUploadDescriptor(
+                    url=AnyUrl(f"https://example.com/{id}/upload"),
+                    method="PUT",
+                    headers={"X-My-Header": "my-value"},
+                ),
             )
         else:
             attachment = FileAttachment(
                 id=id,
                 mime_type=input.mime_type,
                 name=input.name,
-                upload_url=AnyUrl(f"https://example.com/{id}/upload"),
+                upload_descriptor=AttachmentUploadDescriptor(
+                    url=AnyUrl(f"https://example.com/{id}/upload"),
+                    method="PUT",
+                    headers={"X-My-Header": "my-value"},
+                ),
             )
         self.files[attachment.id] = attachment
         return attachment
@@ -679,6 +688,7 @@ async def responder(
         assert events[1].type == "thread.item.done"
         assert events[1].item.type == "assistant_message"
 
+
 async def test_respond_with_tool_status():
     async def responder(
         thread: ThreadMetadata, input: UserMessageItem | None, context: Any
@@ -1019,9 +1029,12 @@ async def test_create_file():
         assert attachment.mime_type == file_content_type
         assert attachment.name == file_name
         assert attachment.type == "file"
-        assert attachment.upload_url == AnyUrl(
+        assert attachment.upload_descriptor is not None
+        assert attachment.upload_descriptor.url == AnyUrl(
             f"https://example.com/{attachment.id}/upload"
         )
+        assert attachment.upload_descriptor.method == "PUT"
+        assert attachment.upload_descriptor.headers == {"X-My-Header": "my-value"}
         assert attachment.id in store.files
 
 
@@ -1049,9 +1062,12 @@ async def test_create_image_file():
         assert attachment.preview_url == AnyUrl(
             f"https://example.com/{attachment.id}/preview"
         )
-        assert attachment.upload_url == AnyUrl(
+        assert attachment.upload_descriptor is not None
+        assert attachment.upload_descriptor.url == AnyUrl(
             f"https://example.com/{attachment.id}/upload"
         )
+        assert attachment.upload_descriptor.method == "PUT"
+        assert attachment.upload_descriptor.headers == {"X-My-Header": "my-value"}
 
         assert attachment.id in store.files