Navigate to the 'File Attachments' section of the Document.

Mouse over the attachment you would like to remove and click the delete icon as shown below.

Confirm that you want to delete the file from the document.

The file has now been successfully removed from the document.

Note: Removing the file from the document does not remove it from the project explorer, to remove files from the project follow this tutorial: How to Remove a File from the Project Explorer.