output, err := client.RunWithOptions(ctx, "black-forest-labs/flux-schnell", input, nil, WithFileOutput())
reader, ok := output.(io.ReadCloser)
if ok {
    defer reader.Close()
    // Read from the reader
    data, err := io.ReadAll(reader)
    // Or stream the data
    _, err := io.Copy(destinationWriter, reader)
}