uploadFile(variables: Record<string, any>, onProgress=(e: ProgressEvent)=>{}): [XMLHttpRequest, Promise<any>] {
        const form = this.client.toFormData(actions.uploadFiles, variables)

        const xhr = new XMLHttpRequest()
        xhr.open('POST', this.url, true)

        return [xhr, new Promise<XMLHttpRequest>((resolve, reject) => {
            for (let [k, v] of Object.entries(this.auth))
                xhr.setRequestHeader(k, v || '')
            xhr.onload = () => resolve(xhr)
            xhr.onerror = () => reject(xhr)
            xhr.onabort = () => reject(xhr)
            if (onProgress)
                xhr.upload.onprogress = onProgress
            xhr.send(form)
        }).then(response => prepare(JSON.parse(response.responseText)))]
    }

toFormData(query: DocumentNode, variables: Record<string, any>) {
        const files = extractFiles(variables, 'variables')
        // GraphQL multipart request spec:
        // https://github.com/jaydenseric/graphql-multipart-request-spec
        const form = new FormData()
        form.append('operations', JSON.stringify({query: print(query), variables}))
        form.append('map', JSON.stringify(
            files.reduce((map: any, {path}, index) => {
                map[index.toString()] = [path]
                return map
            }, {})
        ))
        files.forEach(({file}, index) =>
            form.append(index.toString(), file, file instanceof File ? file.name : 'Blob')
        )
        return form
    }