--- freedesktop.org.xml.in
+++ freedesktop.org.xml.in
@@ -1843,12 +1843,13 @@
     <_comment>MPEG-4 audio</_comment>
     <alias type="audio/x-m4a"/>
     <magic priority="50">
-      <match type="string" value="ftypM4A" offset="0"/>
+      <match type="string" value="ftypM4A" offset="4"/>
     </magic>
     <glob pattern="*.m4a"/>
   </mime-type>
   <mime-type type="video/mp4">
     <_comment>MPEG-4 video</_comment>
+    <sub-class-of type="video/mpeg"/>
     <magic priority="50">
       <match type="string" value="ftypisom" offset="4"/>
       <match type="string" value="ftypmp42" offset="4"/>