**Regression: Drag & drop of an Outlook attachment into the Thumbnails pane fails with "Invalid parameter"**

[RKraus]

Since a recent build, dragging an email attachment directly from Outlook into the Thumbnails (page preview) pane of an already-open document fails with:

> The document cannot be opened.
> An error occurred while opening the document from file: [GUID].jpg
> Error [System]: Invalid parameter.

The GUID-named temp file referenced in the error is Outlook's virtual attachment placeholder. This used to work fine before the update — it would insert the attachment as a new page at the drop location.

**Key diagnostic to narrow this down:**

- Dragging the **same** attachment from Outlook into the **main document area** works correctly (opens it in a new tab / new document).
- Dragging a file from **Windows File Explorer** into the **Thumbnails pane** also works correctly (inserts as a new page).
- Only the combination **Outlook attachment → Thumbnails pane** triggers the error.

This isolates the regression to how the Thumbnails-pane insert handler treats the drop source. Explorer provides a real filesystem path (CF_HDROP), whereas Outlook provides a virtual file via the OLE FileGroupDescriptor / FileContents formats (an in-memory stream with no path on disk yet). The main-frame "open" path still materialises that stream correctly; the Thumbnails-pane "insert page" path no longer does and instead receives an invalid/empty path, hence ERROR_INVALID_PARAMETER.

The attachment type does not matter — it occurs with both PDF and image attachments.

Could the dev team check the OLE virtual-file (FileContents stream) handling in the Thumbnails-pane drop/insert handler? Happy to provide a build number, OS details, or a capture if useful.

Environment:
- PDF-XChange Editor 11.0.1 build: Jun 08 2026; 22:26

ErrorMessage.pdf

- Windows 11 Pro
- Microsoft Outlook (Classic)