Graphql-upload with validation in shema
Hello im sorry for my bad english im french.

Usually I check the inputs directly in my shemas, with type-graphql and class-validator.
But since the file seems to be injected automatically, I don't see how to do it.

Something like that would be possible ?
```
export class UploadImage {
    @IsIn([
        'image/jpeg',
        'image/png',
        'image/gif',
    ])
    @Field()
    public mimetype: string;

    @IsUniqueImage()
    @Field()
    public filename: string;

    @Field()
    public encoding: string;

    @Field(type => Function)
    createReadStream: () => Stream;
}
```

Thanks you :)

