Avoid "ERROR [alfresco.webdav.protocol] [http-8080-1] java.lang.NullPointerException" when browsing a webdav share where the mimetype of a document lacks a human readable version

git-svn-id: https://svn.alfresco.com/repos/alfresco-enterprise/alfresco/HEAD/root@28506 c4b6b30b-aa2e-2d43-bbcb-ca4b014f7261

source/java/org/alfresco/repo/webdav/GetMethod.java

@@ -468,6 +468,10 @@ public class GetMethod extends WebDAVMethod
                     if (reader != null)
                     {
                         mimetype = mimeTypeService.getDisplaysByMimetype().get(reader.getMimetype());
+                        if (mimetype == null)
+                        {
+                           mimetype = reader.getMimetype();
+                        }
                     }
                     writer.write(mimetype);
                 }