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IMG files
Some distributions are shipped as .img files. These are meant to be written to a USB device for creating installation media.
VirtualBox does not know how to mount these by default.
However, it's possible to convert them to a VDI.
cd ~/Downloads
vboxmanage convertfromraw -format VDI name_of_file.img name_of_file.vdi
via: http://www.kombitz.com/2009/10/06/how-to-mount-an-img-file-on-virtualbox/
Also, be sure to allot at least 8gb of storage space for the new disk image after converting, otherwise there won't be room for the initramfs. I did this via Global Tools in VirtualBox