Manual:$wgJpegTran

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<translate> Images</translate>: $wgJpegTran
Path to jpegtran, for lossless JPEG rotation.
<translate> Introduced in version:</translate>1.21.0 (Gerrit change 44005; git #e288e403)
<translate> Removed in version:</translate><translate> still in use</translate>
<translate> Allowed values:</translate>(absolute path) or false
<translate> Default value:</translate>'/usr/bin/jpegtran'

Details

Path to the jpegtran utility. Doesn't seem to be used for much (as of MediaWiki 1.22), other than to provide lossless rotation functionality in the MediaWiki API (see [1]). jpegtran is not even required for image auto-rotation.