CWL definition modification

Question

[1769mkc]

This is a block of my cwl file which I would like to modify

    output_zipfile:
    label: Bioset Zipfile
    outputBinding:
      glob:
      - "$(get_output_dir())/*.zip"
    type: File[]

Here the argument is Required and Multivalve

I tried with this

output_zipfile:
    label: Bioset Zipfile
    outputBinding:
      glob:
      - "$(get_output_dir())/*.zip"
    type: 
    - 'null'
    - File
    items : File?

Now it turned into Optional and Single value

My goal is getting Optional and Multivalue

Any suggestion or help would be really appreciated