Enable attachments for NotebookMarkdownCellModel

src/Notebook_Processor/notebook_models/notebook_cell_model.py

@@ -85,6 +85,7 @@ class NotebookMarkdownCellModel(BaseNotebookCellModel):
     "Notebook markdown cell."
     """
     cell_type: Literal["markdown"]
+    attachments: Optional[dict] = Field(default=None, description="Media attachments (e.g. inline images), stored as mimebundle keyed by filename.")
 
 
 class NotebookCodeCellModel(BaseNotebookCellModel):