Restricted file extensions

Using Outlook 200 SP3 on my system I am having some issues with Microsofts defination of dangerous files. I often receive MDB files from other users, these are being blocked as they fall under Microsofts level one risk. Under this risk there is no prompting they are simply removed.

How can these be got back? / Or what reg settings could I play with to allow some file extensions to be left from this list?