SAP Business One Service Layer - Upload Attachment - Internal Error (-43)

Question

I'm having an issue uploading an attachment into SAP B1 via the Service Layer. I am able to upload an attachment using the SAP client without issue. I'm using Postman to test sending the request before I embed this into a real application. I've gone over the documentation available in the SAP B1 Service Layer Guide (located here https://help.sap.com/doc/0d2533ad95ba4ad7a702e83570a21c32/9.3/en-US/Working_with_SAP_Business_One_Service_Layer.pdf) on page 106.

Here's what my HTTP request looks like... and it matches what the documentation expects.

POST /b1s/v1/Attachments2 HTTP/1.1
Host: 129.159.XXX.XXX:50000
Content-Type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW
Cookie: B1SESSION=62bbdc3e-d634-11ec-8000-02001703771b; ROUTEID=.node3
Content-Length: 197

----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="files"; filename="JSON-Tshirt-Master Product.txt"
Content-Type: text/plain

(data)
----WebKitFormBoundary7MA4YWxkTrZu0gW

Here's the error that I'm getting which doesn't make sense to me. I can't find anything on this error and I was wondering if anyone had seen something like this before?

{
    "error": {
        "code": -43,
        "message": {
            "lang": "en-us",
            "value": "Internal error (-43) occurred"
        }
    }
}