You can also un-attach any previously added attachments by using the "Modify" function.
1. Go to your post and click on "Modify."
2. Below the posting window, you'll see an area called "Attached:" with a list of all the files you previously added to the post (this area was not present when you created your original post, but appears when you modify it):
3. Un-check the attachment(s) you want to remove by clicking the corresponding check box(es).
4. When you're done, click the "Save" button and the post will now appear without the attachment(s) you un-checked.