In GraphQL, how do I filter specific types from a union?

Question

Let's say that I have this query :

searchMedia(term: "puppies") {
  ... on AudioClip {
    duration
  }
  ... on VideoClip {
    previewURL
    resolution
  }
  ... on Image {
    thumbnailURL
  }
  ... on TextSnippet {
    teaserText
  }
}

But I can let users define which types are returned (e.g. users can specify that they only want audio and video clips, not no text or images). Do I have to create a query for each and every permutations or can I specify some parameter to filter out these types?

Note that I'm using pagination through a MySQL query, so filtering on the client after the results are returned would give unpredictable lists of items.

I was hoping that I could create a filter like

input SearchMediaFilter {
   __typename:String
}

but GraphQL complains that it is an internal property. Has anyone got around this?