Attach: allow adding and removing files when in Post Edit mode

[urbansheep]

When a user edits an existing post, she sould have an option to remove, add (and reorder, if possible) files and images.

Adding files: same as attaching files to the new post.
Removing files:

• Images: click on image summons confirmation “Are you sure you want to get rid of this picture?” → Yes removes image, Cancel dismisses.
• Files: After the file name there should appear a link “Remove”, which also displays an inline confirmation request when clicked “Are you sure you want to get rid of this file?”

Reorder: dragging and dropping files or images in the holding container should change their order of attachment to the post item.