単にページレイアウトにボタンを追加しただけでは表示されませんでした。
以下が参考になりました。
I had the same issue. I was able to upload from desktop, the File button was in Mobile and Lightning Experience Actions, but the File button was not showing in mobile. I was also able to upload files on other objects. Taylor Harper's solution is correct.
In Setup go to Feed Tracking > Your Object > Check Enable Feed Tracking and Save.
The file upload button will now appear when you're on the record.