Incorrect value for inferred type from Eden treaty when returning FormData

[aryzing]

What version of Elysia is running?

1.2.9

What platform is your computer?

Linux 6.8.0-50-generic x86_64 x86_64

What steps can reproduce the bug?

From a handler, return FormData like so:

  .get("/example", async () => {
    const file = new File(["hello world"], "hello.txt", { type: "text/plain" });
    
    const formData = new FormData();
    formData.append("file", file);

    return formData;
  })

What is the expected behavior?

If the inferred type of the response FormData, then the value should be a FormData instance.

What do you see instead?

The value is not a FormData instance. Seems Elysia unwraps the FormData and provides a value that looks like

{
  file: File
}

but the entire object is typed as FormData, which doesn't match the actual value.

Additional information

When returning { file } from the handler, Elysia infers the type as { file: File } which doesn't match the actual value either. It's just an empty object on the client, {}.

Seems the only way to preserve file data on the client and getting the correct types is by returning FormData from the handler, but forcing the correct type on the client,

const file = (res as unknown as { file: File }).file;