Report the real file size of exportable Drive documents

File size is not a well defined concept for special Drive documents (Docs, Sheets, Slides) because it depends on the export format. For such documents, GCSF should report the corresponding file size they would have when exported with the default MIME type.

One way of achieving this is by exporting each document at mount time in order to obtain its real size. This is incredibly slow. For the moment, GCSF reports the maximum file size (10 MB) for all special documents.