Comment 11 for bug 993275

what does these command output for you?

$ xdg-mime query filetype /usr/share/themes/Ambiance/metacity-1/close_focused_normal.png
$ xdg-mime query filetype /usr/share/themes/Ambiance/metacity-1/close.png

over here it says "image/png"

/usr/share/mime/packages/subtitlecomposer.xml contains this:

        <mime-type type="text/x-tmplayer">
                <comment>TMPlayer subtitles</comment>
                <sub-class-of type="text/plain"/>
                <generic-icon name="text-x-generic"/>
                <magic priority="100">
                        <match type="string" value="xx\:xx\:xx\:" mask="0x0000ff0000ff0000ff" offset="0"/>
                        <match type="string" value="xx\:xx\:xx\:" mask="0x0000ff0000ff0000ff" offset="9:256"/>
                </magic>
                <glob pattern="*.sub"/>
        </mime-type>