xz is the successor of lzma, and the file format has now been finalized (see http://tukaani.org/xz/xz-file-format.txt) With distros considering to or already switching to lzma-compressed rpm payload, having this mimetype seems somewhat relevant. The spec says this: This document describes the .xz file format (filename suffix ".xz", MIME type "application/x-xz"). It is intended that this this format replace the old .lzma format used by LZMA SDK and LZMA Utils.
It has been added (by myself) two months ago in CVS. It will be in the next version.
*** Bug 22671 has been marked as a duplicate of this bug. ***
*** Bug 24225 has been marked as a duplicate of this bug. ***
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