Error on the terminal (does show up in the error log):
itk::ExceptionObject (0x7fe3f8e933f8)
Location: "void itk::PNGImageIO::WriteSlice(const std::string &, const void *)"
File: /Users/nicole/Slicer-git/Slicer-build/ITKv4/Modules/IO/PNG/src/itkPNGImageIO.cxx
Line: 519
Description: PNG supports unsigned char and unsigned short

For the Stonybrook project we need to support IO for PNG and/or TIFF. This instance seems to be a case of needing to cast the image to be saved, and to give better feedback to the user.

Digging into this issue, I found one bug in the volumes reader in that it was returning an improperly formatted string list of supported extensions, so PNG files weren't registered as supported.
Another complicating factor is that when you select a PNG file to be loaded, Slicer goes through various volume type readers to try and load it, and the vector file reader succeeds, so the volume node is a vtkMRMLVectorVolumeNode.

This topic branch adds a load, convert to greyscale, cast, edit, save test for a PNG file in the Stonybrook example data set online. It also fixes the volume file extensions bug as well as a copy paste one in the volumes logic:
https://github.com/naucoin/Slicer/commits/Add-png-load-edit-save-reload-test