Mastodon has a post editing feature which lets you make changes to your posts after they've been published.
Here's how to edit:
1. Go to the post
2. Click ⋯
3. Click "Edit"
4. Make the changes you want (including adding or removing attachments, or adding alt text descriptions)
5. Click "Save Changes"
NOTE: To prevent abuse, every time you edit a post it will send a notification to everyone who has interacted with it. So, try not to make too many edits! 😁
You cannot edit post visibility.