Add ability to retrieve internal document types by inferred extension

[tvierling]

So, for a Google Document or Spreadsheet with name "docname", if the user attempts to open "docname.docx" or "docname.xls" or "docname.pdf", the MIME type will be inferred and trigger an export.

The entries should not show up in directory listings, because stat.st_size is not yet known until the export happens. These would effectively be hidden "files" that must be retrieved by name.

[tvierling]

Open question: whether [l]stat() on docname.EXT should succeed. Most programs will likely want this to work, as not all of them use fstat() to get the file size after opening it. This would imply that such a call would trigger an export into the local cache.

Perhaps via a mount option?