If you wanting to attach a document from your Google Drive, you will need to use the "Google Drive" icon rather than the attachment icon (paperclip). However, if you are using a chromebook, clicking the attachment icon will only allow you send a file that is physically on your chromebook....which is usually located in your "Downloads" folder. 


RULE: Always use the "attach" files link to attach files physically on your computer and use the Google Drive button for anything in your My Drive.


Click this link to see the forum post.