Microsoft describe the solution in this KB article and lists all blocked attachments here. As there is no way to say "unblock all" you need to specify every possible type that may be blocked. e.g.
Office 2007 Example (copy/paste into a .reg file)
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Security]
"Level1Remove"=".ade;.adp;.app;.asp;.bas;.bat;.cer;.chm;.cmd;.cnt;.com;.cpl;.crt;.csh;.der;.exe;.fxp;.gadget;.hlp;.hpj;.hta;.inf;.ins;.isp;.its;.js;.jse;.ksh;.lnk;.mad;.maf;.mag;.mam;.maq;.mar;.mas;.mat;.mau;.mav;.maw;.mda;.mdb;.mde;.mdt;.mdw;.mdz;.msc;.msh;.msh1;.msh2;.mshxml;.msh1xml;.msh2xml;.msi;.msp;.mst;.ops;.osd;.pcd;.pif;.plg;.prf;.prg;.pst;.reg;.scf;.scr;.sct;.shb;.shs;.ps1;.ps1xml;.ps2;.ps2xml;.psc1;.psc2;.tmp;.url;.vb;.vbe;.vbp;.vbs;.vsmacros;.vsw;.ws;.wsc;.wsf;.wsh;.xnk;"
Microsoft describe the solution in this KB article and lists all blocked attachments here. As there is no way to say "unblock all" you need to specify every possible type that may be blocked. e.g.
Office 2007 Example (copy/paste into a .reg file)
See this solution (http://ask-leo.com/how_do_i_force_outlook_to_open_potentially_unsafe_attachments_that_i_know_are_ok.html)