Using Outlook it's possible that the attachment received are in grey color.
To solve this problem we must use:
start --> run ... --> regedit.
looking for this section:
Microsoft Office Outlook 2013HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Security
Microsoft Office Outlook 2010HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Security
Microsoft Office Outlook 2007HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Security
Microsoft Office Outlook 2003HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Security
Microsoft Outlook 2002
HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Security
Microsoft Outlook 2000
HKEY_CURRENT_USER\Software\Microsoft\Office\9.0\Outlook\Security
When we have found our section we must create a new STring Level1Remove and we should insert the file extension that we want enable to download (For example .exe, .com, .vbs)