Correct JPEG2000 definition

*.jpc and *.j2k relate to JPEG 2000 codestreams, not to JPEG 2000
images, so remove those globs.

Remove the jP magic, as it creates false positives (and add a test for
that). Also add the jp2 string magic.

With help from Steve Fosdick <other@fosdick.me.uk>

https://bugs.freedesktop.org/show_bug.cgi?id=11942

freedesktop.org.xml.in

@@ -3969,16 +3969,17 @@ command to generate the output files.
     <_comment>JPEG-2000 image</_comment>
     <alias type="image/jpeg2000"/>
     <alias type="image/jpx"/>
+    <alias type="image/jpeg2000-image"/>
+    <alias type="image/x-jpeg2000-image"/>
     <magic priority="50">
-      <match value="jP" type="string" offset="4" />
       <match value="\xFF\x4F\xFF\x51\x00" type="string" offset="0" />
       <match type="big32" value="0x0c6a5020" offset="3"/>
+      <match type="string" value="jp2" offset="20"/>
     </magic>
     <glob pattern="*.jp2"/>
-    <glob pattern="*.jpc"/>
     <glob pattern="*.jpx"/>
-    <glob pattern="*.j2k"/>
     <glob pattern="*.jpf"/>
+
   </mime-type>
   <mime-type type="image/openraster">
     <_comment>OpenRaster archiving image</_comment>

tests/list

@@ -25,7 +25,8 @@ test.ilbm image/x-ilbm
 test.im1 image/x-sun-raster x
 test.jp2 image/jp2
 test.jpg image/jpeg
-test.jpc image/jp2
+# Possible false positive for a JPEG 2000 image
+img_5304.jpg image/jpeg
 # From https://bugzilla.freedesktop.org/show_bug.cgi?id=55339
 test.mng video/x-mng
 test.pat image/x-pat xxx