Preview priority of uploaded files

A file can be set as the preview for the uploaded dataset. Currently, NekUpload selects it at random, or maybe it takes first one in the list to preview. This should be set properly to ensure consistency on what other users will see. The following are suggestions:

• Simulation record could be previewed with session file or metadata.json
• Geometry record could be previewed by an image or metadata.json

Avoid setting a HDF5 binary file as preview as it would be useless.