Add extensions files for H5Viewer

Current list in data portal is ['.h5', '.nxs', '.nx', '.hdf5']
In JLab-h5web: ['.hdf5', '.h5', '.nexus', '.nx', '.nxs', '.cxi']
In silx [".nx", ".nx5", ".nxs", ".h5", ".hdf", ".hdf5", ".cxi"]

=> it should be updated to ['.h5', '.nxs', '.nx', '.hdf5', '.nexus', '.cxi', '.nx5', '.hdf']