Since Atom 1.6 , this behavior is built into Atom itself and is independent of the package of tabs or external. “Use preview tabs” (as shown in the accepted answer) is no longer available .
It is now called "Pending Pane Items" and is described in the Atom 1.6 Release Announcement .
Sometimes it’s useful to take a look at the contents of a file without opening it in a separate tab. For these cases, you want pending panel elements.
The differences between the now-called “pending panel elements” and “preview tabs” were also discussed on the Atom platform. Discuss: Pending panels and preview tabs .
Pending panel elements are now enabled by default, and there is no need to activate anything to use it. For those who do not like this, you can disable it from the "Basic Settings" by unchecking the "Allow pending panel items" checkbox:

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