SSDM-2017 Performance improvements in thumbnail generation : bug fix - release file handle

SVN: 34329

datastore_server/source/java/ch/systemsx/cisd/openbis/dss/generic/shared/utils/ImageUtil.java

@@ -500,6 +500,10 @@ public class ImageUtil
                 return;
             }
             filePath = newFilePath;
+            if (handle != null)
+            {
+                closeQuietly(handle);
+            }
             handle = contentNode.getFileContent();
         }