This is a very simple page. This is the Attachment section. These are the file types that if they are attached to your e-mail, the extension will get replaced. The attachment is still runnable but when you try to run it you will get the following popup.
If you still want to run the attachment, then click the Run button. Or you can click the Save button and the attachment will be saved to whatever location you tell it. Then you can do whatever with the file, scan it for virus's, Trojans, malware, or whatever else you want to. You can also click the Inspect with Notepad button, in which case the file will open in notepad, no matter what kind of file it is. Then you will be able to inspect the code. For most users this will just look like garbage. But to the trained eye, it may be possible to see a signature of a malware writer. And the last option is to click the Do Not Run button. This will close the file and take you back to your e-mail client.
If you want to add an attachment type, click the Add button, you will see the following window.
You enter in anything you want for an description, and then whatever extension you want. It needs to be in the form of *.xxx just replace the xxx with whatever you want.