Certificate (app-only) auth works, but “Add attachment to list” fails with 401 (SaveBinary) — anyone else?

Hi all,
I’m running into a strange issue with SharePoint integration in WEBCON and wanted to check if anyone else has seen something similar.
I’m using Azure App with certificate-based (app-only) authentication (SharePoint API, Application permission: Sites.ReadWrite.All with Admin Consent, not delegated).

✅ What works

Connection to SharePoint works
General communication works
Folder creation in document library works


❌ What doesn’t
When using “Add an attachment to a list”, the file upload fails with:
401 Unauthorized


🔍 Logs / details
From WEBCON:
Execute WCFCsomService .../AddAttachmentToSPList
Error in On path action Add attachments to Extranet (Step: Signing → Prepare for mailing)
The remote server returned an error: (401) Unauthorized.

Stack trace:
System.Net.WebException: (401) Unauthorized
at Microsoft.SharePoint.Client.SPWebRequestExecutor.Execute()
at Microsoft.SharePoint.Client.File.SaveBinary(...)
at WebCon.WorkFlow.CSOM.Sharepoint.AddAttachmentToSPListCsomProxy.AddToDocumentLibrary(...)


🧠 Observations

Authentication clearly works (since folder creation succeeds)
The error happens specifically at:
File.SaveBinary()


So it looks like the issue is tied specifically to the file upload step


⚠️ Important detail
If I switch to a service account (username/password), the exact same action works perfectly and uploads files without any problems.
However, due to upcoming Microsoft authentication policy changes, we must use certificate-based auth only (introduced in newer WEBCON versions like BPS 2026).

❓ Questions

Has anyone successfully used “Add attachment to list” with certificate (app-only) auth?
Any recommended workaround (without switching auth type)?


Any insights or confirmations would be really appreciated 👍