Persisting media attachments BUG

[andreyasko]

The issue of media attachments persisting across unsubmitted forms is significant and needs comprehensive solution.

1. When I upload an attachment to post form, then close it (don't post) and try to post again, the attachment is still there.
2. Intermittently attachments from other accounts/profiles show up.

Any unsubmitted attachment should be deleted/purged if the form is closed and not submitted.

[AntonLV]

Duplicate for issue #4430