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:

1. Type Command-Shift-G while in the Finder.
2. Type or copy the following /System/Library/CoreServices and push Enter
3. Right-click on CoreTypes.bundle and select Show Package Contents
4. Open Contents, then Resources.
5. Download the following linked file, double-click it to unzip it, then drag the file XProtect.meta.plist into the Resources folder. Download File
6. Enter your administrator username and password.
7. Restart your computer.

You should now be able to see your attachments in Mac Mail!