To find out about how/why of blocked extensions read
this post 
Try a clean install of MSNM:
- uninstall MSNM
- reboot
- remove all folders pointing towards MSNM
- reinstall MSNM
After that you can unblock the blocked extensions again.
Possibilities to unblock are:
File Extension Unblocker
A-Patch
Mess Patch 7.0.0777
SPNG (Msg Plus! required)
Please note that these filetypes were blocked in order to counter the propagation of viruses over the .NET Messenger network. Always be wary of any file you receive!
Advice: Scan the files you receive with a good AV program (also make sure the virusdefinitions are up to date) after receiving first. You can set MSNM to do that: tools > options > file transfer.
/ 
