You can attach files to a message at any time. You can attach any type of file, including documents, images, or videos, as long as it’s under 5 GB.
Files can come from your computer, or from cloud storage services like Google Drive, OneDrive/SharePoint, Box, and Dropbox. The processes for attaching a file from your computer, Google Drive, OneDrive/SharePoint, Box, or Dropbox are all a little different.