Right click an attachment and choose "Drop" from the pie menu
Unlike in previous versions of Second Life, the attachment isn't automatically selected when dropped.
Prior to 1.7 the avatar would point at the dropped attachment and the selection particles would stream towards the dropped attachment (even if the edit window was not open when the attachment was dropped). Once the edit window was opened (or if the edit window was already open) the dropped attachment would be selected for editing.
This is important when working with small or transparent attachments (any attachment that would be hard to find once dropped).
Selecting the attachment once dropped helps locate the dropped attachment and allows you to immediately begin editing rather than having to pan/zoom the camera to locate it.
I've bug reported this because I'm assuming it's just an oversight.