Here's a photo for you Roman.
I used "Go Advanced" button at the end of the thread, entered the text of the post, and used "Manage Attachments". You might want to open the "Manage Attachments" window in a new tab, instead of in a new window, particularly if you have Java and JavaScript disabled...
And then use "Preview Post" button to make sure it's attached - otherwise you won't see it, and won't know whether it's attached or not.
Denis
PS: I'm using Mozilla under Linux - I noticed it won't work as usual if I disable Java and javaScript, which I did a few days ago, due to a security issue/vulnerability with hujacking JavaScript input boxes 🙁
Anyway, the above process indeed works, as the photo proves 🙂