Disable Attachment Blocking (Outlook)
Outlook by default blocks certain attachments (file-types) from being viewed for security reasons. Once blocked the file cannot be 'un-quarantined' and it can be more useful to disable the blocking at the client-level and establish filtering at another level.
To disable blocking on a per-extension basis:
Locate the following registry key, where <version> is your version of Outlook:
Create a new string key called "Level1Remove" and add the extnsion(s) you wish to allow; seperating them with commas. Outlook will need to be restarted for the changes made to take effect.