Click Google Drive . Select the files you want to attach. At the bottom of the page, decide how you want to send the file: Drive link: This works for any files stored in Drive, including files created using Google Docs, Sheets, Slides, or Forms. Attachment: This only works for files that weren't created using Google Docs, Sheets, Slides, or Forms. read more
On the new compose window (pop up) click on the Drive icon to insert a file as attachment. Step 2: The file selection window will open up. Here, you can choose a file that is already a part of your Google Drive’s space or you can choose to upload one to the same. read more