Type with name "Upload" does not exists

Hi there,

I'm continually getting this error when I'm trying to use `Upload` scalar within my schema. I've followed and read a ton of posts and SO links but can't seem to get past this.

```sh
3|server:watch | throw new Error(`Type with name ${(0, _misc.inspect)(typeName)} does not exists`);
3|server:watch | ^
3|server:watch | Error: Type with name "Upload" does not exists
3|server:watch | at SchemaComposer.get (/Users/andrewmartin/code/server/node_modules/graphql-compose/lib/TypeStorage.js:44:13)
```

This happens when I try to add the field directly like so:

```ts
UserTC.addFields({
image: 'Upload',
})
```

I'm trying to add an `Upload` type. This type, by the way, *works successfully* against my schema and in non-compose / TC resolvers:

```buildSchema.ts
const schemaPath = join(__dirname, '../../../src/gql/schema/**/*.graphql')

const schemaFiles = loadSchemaSync(schemaPath, {
loaders: [new GraphQLFileLoader()],
})

logger.info(`loaded schema! from path: ${schemaPath}`)

const uploadSchema = makeExecutableSchema({
typeDefs: `scalar Upload`,
resolvers: { Upload: GraphQLUpload },
})

const schemaComposer = new SchemaComposer(uploadSchema)

export const schemaWithResolvers = addResolversToSchema({
resolvers,
schema: schemaFiles,
})

schemaComposer.merge(schemaWithResolvers)

schemaComposer.Query.addFields({
...UserQuery,
...PageQuery,
...MenuQuery,
})

schemaComposer.Mutation.addFields({
...adminAccess(UserMutation),
...adminAccess(PageMutation),
...MenuMutation,
})

schemaComposer.merge(schemaWithResolvers)

export const schema = schemaComposer.buildSchema()
```

Main part of interest is here:

```ts
const uploadSchema = makeExecutableSchema({
typeDefs: `scalar Upload`,
resolvers: { Upload: GraphQLUpload },
})
```

I've seen some weird workarounds people are doing to add it successfully when `Mutation.addFields` is hit, but that won't work well for my case because I want to simply add a new argument onto the types available. Any workarounds?

Thanks in advance for your help.