Attach: defer loading of attached files until New Post has been submitted

[urbansheep]

For consideration: old FRF didn't upload files until post was submitted, so all local files and pictures were displayed in the preview but didn't get uploaded until user pressed “Post” button. This might be a sound idea to decrease incoming transfers and bind them to post event, so that user wouldn't be able to spam server by adding/removing files or pictures indefinitely.