Flex: fix support for new file naming scheme

See http://lists.openmicroscopy.org.uk/pipermail/ome-devel/2014-March/002721.html and https://trac.openmicroscopy.org.uk/ome/ticket/11382.

Recently acquired plates use one file per field, where previously fields were typically stored within the file.  To test, verify that the dataset listed in the ticket has 4 wells, 6 fields, and 30 planes per field - the planes should also visually look as though they all belong in the same field.