The default e-mail client in OS X is Apple's Mail program, which supports a feature that permits you to view many common attachment file types in-place instead of having to download and open them in another program. For example, if someone sends you a picture you should be able to view it directly in Mail. However, for some people this may not be the case and instead of showing an attachment, Mail will display a gray box that reads "Missing Plug-In."
If this happens to you, the first thing to do is troubleshoot the Internet plug-ins on your computer. These ins are located in two folders, both called "Internet Plug-Ins," that are located in the global library at the root of your hard drive and in the library in your user account folder. To get to the user library, choose "Library" from the Go menu in the Finder (hold the Option key to reveal this menu entry if it is missing).
In these folders, locate and move any Java-related plug-in, which can be called "JavaPluginCocoa.bundle" or "JavaAppletPlugin.plugin," to another location, such as your desktop. After these have been removed, relaunch Mail to see if the problem persists. If so, then return these plug-ins to their original location and try this same routine with the other plug-ins until you are able to pinpoint the problematic one.
In addition to Internet plug-ins, this issue may revolve around corruption of a Java-related preferences file, even if your system does not have Java installed or enabled. To tackle this, open the Preferences folder in your user library (accessed from the Finder's Go menu), then locate and remove the file called "com.oracle.java.JavaAppletPlugin.plist" along with any other files that begin with the name "com.oracle.java." Then try relaunching Mail to see if the problem persists.