Office 365 (Outlook 2013 / 2016 for Win) - Delete an Attachment From an Email
1. Select the attachment by left-clicking on it once. This will allow you to preview it, if desired. If you don't want to preview it first, skip to the note at the bottom of this article.
2. Select Remove Attachment from the ribbon.
NOTE: If you do not wish to preview the attachment first, simply right-click on the attachment once and select Remove Attachment from the popup window.