Unfortunately, the screenshot you mentioned in your question is not displayed.
Which step processor are you planning to customize?
Is this for the IBM FileNet Content Manager (P8) or iBM Content Manager (CM8) content repository?
In the P8 OOTB step processors, the user should manually remove any documents or folders from the attachments that are exposed at the current step. This is performed via the P8WorkAttachmentToolbar actions, which make changes to the ContentList. This will call the _updateFromContentItem() function in WorkItem.js.
Please note that the user will not be able to edit any read-only attachment parameters. If your custom step processor updates read-only step parameters, then an error will be returned when you try to save the changes.
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