Fix suggested by dosumis

To fix issue not finding metadata for expression patterns displayed on
the viewer

http://www.virtualflybrain.org/site/tools/view_stack/3rdPartyStack.htm?j
son=Janelia2012/Brain/GMR_12D10_AE_01_08-fA01b/wlz_meta/tiledImageModelD
ata.jso&type=THIRD_PARTY_STACK&tpbid=VFB_00022619