The file extension of any unknown mimetype is lost during download, and replaced by .bin.
Steps to Reproduce:
1. Login to Alfresco Share as a user (admin or non-admin)
2. Create a public collaboration site
3. Upload a MS Access database file (*.accdb) to that site
4. Click on the document name to go to the View Details page then click Download
The *.accdb file ought to be downloaded, keeping the file extension.
The *.accdb file extension is lost and replaced by .bin. See accdb-as-bin.png screenshot.
The problem does NOT occur with a Firefox client.
This seems similar to MNT-11134. I suspect that it happens because *.accdb is an unknown mime type.