If you're wondering which applications are currently being used to preview files and want to manage individual previewers, you can click on the "Attachment and Document Previewers" button. Here you can selectively enable or disable previewers:

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For example, if you disable the Microsoft Office Word Previewer, you won't be able to preview the Word documents in your e-mail message. Instead, you'll see the message, "This file cannot be previewed because the following previewer has been disabled: Microsoft Office Word previewer." You'll also see a button underneath the message that gives you the option to "Enable the previewer."

Keep in mind that Attachment Preview is supported for messages that are formatted with HTML or plain text only. It's not supported for messages that are formatted as Rich Text Format (RTF). Also, you can only preview attachments when you're looking at received messages; you can't preview an attachment in a message that you've composed.