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.