The syntax for renaming files after downloading, e.g. SRC_URI="http://someurl/inadequate-name-${PV}.wrong_extension -> ${P}.tar.gz" is not documented in the SRC_URI section of the document found here: http://devmanual.gentoo.org/ebuild-writing/variables/index.html#src_uri
It is documented here: http://devmanual.gentoo.org/ebuild-writing/eapi/index.html
*** Bug 377319 has been marked as a duplicate of this bug. ***
Reassigning to devmanual
Fixed in https://gitweb.gentoo.org/proj/devmanual.git/commit/?id=fc94b7c5ee7672096abbfead15f956b887c6d5ba