Question about 'Upload value invalid' error.
Hi! I'm sorry if this is not the best way for adding this, please direct me to a better place if that's the case. I followed your implementation for adding Upload capabilities to an existing application. The code uses the `GraphQLUpload` and `FileUpload` classes, and the mutation looks something like this:

```js
@typeGraphql.Mutation(() => Boolean)
    async addProfilePicture(
        @typeGraphql.Arg("picture", () => GraphQLUpload)
        {
        createReadStream,
        filename
        }: FileUpload): Promise<boolean> {
      return new Promise(async (resolve, reject) =>
        createReadStream()
          .pipe(createWriteStream(__dirname + `/../../../images/${filename}`))
          .on("finish", () => resolve(true))
          .on("error", () => reject(false))
      );
    }
```

In Postman I have the following body:

```
operations:{"query":"mutation addProfilePicture($picture: Upload!) {addProfilePicture(picture: $picture)}", "variables": {"picture": null}}
map:{"0":["variables.picture"]}
```

and a local file added in the '0' field.

when running a query with Postman, I get the error:

```
Variable \"$picture\" got invalid value {}; Expected type Upload. Upload value invalid
```

I've been at this for 3 days already. could use a bit of help..