MAIL ATTACHMENTS SHOWING ONLY AS BLOCKED PLUGIN
The reason for this problem occurring is due to a missing file in your computer. The file is called XProtect.meta.plist and should reside in your System -> Library -> CoreServices -> CoreTypes.bundle -> Contents -> Resources folder.
I am uncertain as to why this file is removed or disappears, but it is necessary for Mail to recognize the types of files and allow you to view them in your email. It doesn't prevent you from downloading the attachments, but can get really annoying.
In order to get there, follow these steps:
You should now be able to see your attachments in Mac Mail!