Keep existing attachments

Author: Jürgen Hasch

src/jupyter_contrib_nbextensions/nbconvert_support/pre_embedimages.py

@@ -104,7 +104,7 @@ def preprocess_cell(self, cell, resources, index):
             Index of the cell being processed (see base.py)
         """
         self.path = resources['metadata']['path']
-        self.attachments = Struct()
+        self.attachments = getattr(cell, 'attachments', Struct())
 
         if cell.cell_type == "markdown":
             regex = re.compile('!\[([^"]*)\]\(([^"]+)\)')