title: Graphql Apollo upload in Nestjs returns invalid value {}
body: <p>I tried adding an upload argument to a GraphQL endpoint using <code>graphql-upload</code>'s <code>GraphQLUpload</code> scalar:</p>
  
  <pre class="lang-js prettyprint-override"><code>import { FileUpload, GraphQLUpload } from 'graphql-upload'
  
  @Mutation(() =&gt; Image, { nullable: true })
  async addImage(@Args({name: 'image', type: () =&gt; GraphQLUpload}) image: FileUpload): Promise&lt;Image | undefined&gt; {
  // do stuff...
  }
  </code></pre>
  
  <p>And this worked initially. A few runs later however, and it started returning the following error:</p>
  
  <pre><code>"Variable \"$image\" got invalid value {}; Expected type Upload. Upload value invalid."
  </code></pre>
  
  <p>Tried testing with Insomnia client and curl:</p>
  
  <pre class="lang-sh prettyprint-override"><code>curl localhost:8000/graphql \
    -F operations='{ "query": "mutation ($image: Upload!) { addImage(image: $image) { id } }", "variables": { "image": null } }'
    -F map='{ "0": ["variables.image"] }'
    -F 0=@/path/to/image
  </code></pre>