Add initial file uploads extension draft

.github/CODEOWNERS

@@ -0,0 +1 @@
+* @modelcontextprotocol/file-uploads-wg @modelcontextprotocol/core-maintainers

README.md

@@ -1,2 +1,43 @@
 # experimental-ext-file-uploads
-Experimental MCP extension for uploading files to tools over MCP. Maintained by the File Uploads Working Group.
+
+Experimental MCP extension for moving files between a client and a tool out of band, so the bytes
+stay out of model context and out of JSON-RPC messages. Maintained by the
+[File Uploads Working Group](https://modelcontextprotocol.io/community/working-groups/file-uploads).
+
+> [!IMPORTANT]
+> This is an [experimental extension](https://modelcontextprotocol.io/extensions/overview#experimental-extensions).
+> It is not part of the MCP specification and has not been proposed as a SEP. Expect breaking
+> changes while the working group iterates.
+
+## Overview
+
+A server marks file arguments in a tool's input schema. Each file argument contains its metadata
+and optional base64 data. When data is absent, the server rejects the call or requests an upload.
+The client uploads the bytes and retries with unchanged arguments. A tool returns a file by listing a download target in the result's `_meta`.
+
+The draft is in [`specification/draft/file-uploads.md`](specification/draft/file-uploads.md).
+The reference implementation will live in this repository.
+
+## Background
+
+- [Working group meeting notes, October 2, 2026](https://github.com/modelcontextprotocol/modelcontextprotocol/discussions/3412)
+- [SEP-2356: Declarative File Inputs](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2356)
+  (closed), source of the `x-mcp-file` keyword and inline transfer
+- [SEP-2631: File Objects and Transfer](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2631)
+  (closed), prior art for transfer descriptors
+- [SEP-2532: Resource Streaming](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2532),
+  related work on getting file bytes from a server
+
+## Participating
+
+The working group meets weekly and discusses this work in its thread on the
+[MCP Discord](https://modelcontextprotocol.io/community/communication#discord). Open an issue or
+pull request here for anything specific to the draft.
+
+We want to hear whether the flow fits your storage backend's upload API, which
+[open questions](specification/draft/file-uploads.md#open-questions) block you, and whether
+anything in the security model would stop you from deploying it.
+
+## License
+
+Apache 2.0. See [LICENSE](LICENSE).