Adding the video/webm mime type for files with extension .webm. The change is...

Adding the video/webm mime type for files with extension .webm. The change is similar to JENKINS-3803, just a different extension.

war/src/main/webapp/WEB-INF/web.xml

@@ -194,5 +194,9 @@ THE SOFTWARE.
     <extension>rar</extension>
     <mime-type>application/octet-stream</mime-type>
   </mime-mapping>
+  <mime-mapping>
+    <extension>webm</extension>
+    <mime-type>video/webm</mime-type>
+  </mime-mapping>
 
 </web-app>