Add an Attachment
Click the Add or Upload Files button.
If you choose to Upload a file that isn’t in your Media Library, you can either select it via the handy little Select Files button, or you can drag the files from a folder and drop them here.
Once the file is uploaded, it will be added to your Media Library and (1) automatically selected with a check mark. If it’s the file you want, (2) click Use This File. If you want (3) to Edit or Delete the file, there’s options for that in the upper right corner.
And obviously, if you want to upload a file that is already in your Media Library, click on the file and click Use This File.