Make the "Open" filebrowser context menu item work with ".hdf5" files

[telamonian]

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I'm currently exploring some changes to core to make this happen. Some possibilities:

• Introduce a dirlike filetype to go alongside directory and file in the core contents machinery. When you open a dirlike in the default filebrowser, it will switch to a particular browser, then use that browser's drive to cd to dirlike.path.

• add contents handling callback to the optional filetype parameters. At some point in the 'filebrowser:open' command, in between getting an item and passing item.path to the next open command, have a hook for a callback like:

  (item: Contents.IModel): string => {
    <do stuff>
  
    return path;
  }