Copy attachment from a list item to another list item using SharePoint REST API

I am having trouble using SharePoint's REST API to copy attachments from one list item to another list item in a different site collection. Can someone please help?

Using REST, I am able to generate the attachment, but it's contents are blank

I used a Web Request to get the contents of the attachment:

https://<site>/_api/web/getfilebyserverrelativeurl('<relativeURL>')/$value?binaryStringResponseBody=true

I then used another Web Request to add the attachment to desired list item:

https://<site>/_api/web/lists/GetByTitle('')/items({WorkflowVariable:itemID})/AttachmentFiles/add(FileName='<filename>')

However, the attachment is blank :(

I know there have been previous topics that have touched on how to do this, but I am just not able to piece together the bits to get it working.

Some of the posts and blogs I have been looking at include:

Fetching an Adding an attachment using Call Web Service action

https://community.nintex.com/t5/Nintex-for-SharePoint/Fetching-an-Adding-an-attachment-using-Call-Web-Service-action/td-p/3099

How to execute a SP2013 REST API request with Nintex Workflow

https://community.nintex.com/t5/Community-Blogs/How-to-execute-a-SP2013-REST-API-request-with-Nintex-Workflow/ba-p/81942

Thanks in advance :)