To the left of your share button in the top right-hand corner is your attachment(s) icon, shown as a paper clip. Click on the to be brought directly to your Attachments.
There are two types of attachments:
A track attachment is a great space to add stems, a lyric document, or reference tracks for that specific uploaded track.
As shown above, a Space Attachment is a great place to add any external links, album artwork, a reference track/playlist, YouTube video, or a spreadsheet that discusses splits or the project.