I highy doubt they're encrypted. What makes you believe they are?
I think, instead, that you see the MIME *encoding* of the attachment because
something is preventing Outlook from seeing the message data correctly so that
it can decode it properly into the original. The usual cause of this is
scanning your mail with an antivirus scanner as it arrives or as you send
messages. Uninstall your AV program and reinstall it without the mail
scanning feature. You will not be compromising your safety in any way.
