Hi there,
is there a possibility to define a folder as the default folder to look for attachments?
Thanx in advance

Yes.

• When in the attachment pane, click the "Add attachment..." button
• Use that file browser to open the folder you want to set as default
• Click the "Set as favourite dir." button.

From then on you will be able to use the "Go to favourite dir." button for single click navigation to that folder.