You can dig through Outlook's menus when composing an e-mail should you want to add attachments, but if you already have a file pulled up in Windows Explorer, you can easily send it over to Outlook in a new message.
- Right-click on the file you want to attach.
- Click Send To > Mail Recipient.
- The file will be added to a new message. An automated text will appear in the body of the e-mail, but you can quickly delete it by pressing Ctrl + A > Backspace.

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