8 – How to have a button to open a modal popup from a table form?

I have a custom entity (contract_subscription) created in a custom module (amap) by a group of BaseFieldDefinition, one of which is a file field:

$ fields['file'] = BaseFieldDefinition :: create (& # 39; file)
-> setLabel (t (& # 39; pdf file & # 39;))
-> setSettings ([
    'target_type' => 'file',
    'file_extensions' => 'pdf',
    'uri_scheme' => 'private',
    'file_directory' => 'contracts/subscriptions',
  ])

I have created a form for "mass" update this entity.
In this form, the file field is defined as follows:

$ form['subscriptions'][$key]['file']    = table (
& # 39; # type & # 39; => & # 39; managed_file & # 39;
& # 39; # upload_validators & # 39; => array (
& # 39; file_validate_extensions & # 39; => table (& # 39; pdf & # 39;),
)

which gives this:
enter the description of the image here
I would like to replace the standard way in which the file field is presented by a simple button.
Clicking the button will display a modal context menu containing the standard file field management.
How can I do that?