Upload of two and more scalars
Hi! Thanks for repository.

I'm having a problem with uploading of two scalars.

My input with two scalars for mutation is:
```
input createTrainingInput {
  name: String!
  level: TrainingLevel!
  introduction: String!
  workoutDuration: String!
  description: String!
  goals: [String]!
  requirements: [String]!
  overallDuration: Int!
  intensity: Int!
  price: Int!
  promotionalVideo: Upload!
  coverPhoto: Upload!
}
```

When I upload small plain text files, everything is ok.

But when I'm trying to upload video one of the promises is in 'pending' state for a long time, until FileStreamDisconnectUploadError is thrown.

Resolvers code:
```javascript
const { coverPhoto, promotionalVideo, ...properties } = input;

const [
    { stream: coverPhotoStream, filename: coverPhotoFilename },
    { stream: promotionalVideoStream, filename: promotionalVideoFilename },
] = await Promise.all([
    coverPhoto,
    promotionalVideo
]);
```
Version of apollo-upload-server is 5.0.0.

Please, advice what can be done in this situation.

Thank you.
