I guess It's a bit late but I am using this option to preview the files and it works well for me.
Field = Formula URL
var text url = URLRoot() & "up/" & Dbid() & "/a/r" & [Record ID#] & "/e6/v0";
var text doctype = Right(Right(ToText([File]), "/"), ".");
If ($doctype = "pdf" or $doctype = "doc" or $doctype = "PDF" or $doctype = "DOC" or $doctype = "xls" or $doctype = "xlsx" or $doctype = "pptx" or $doctype = "docx", "https://docs.google.com/gview?url="&$url&"&embedded/=true",URLRoot() & "up/" & Dbid() & "/a/r" & [Record ID#] & "/e6/v0")
Places where you have e6 insert the field I.D instead of 6 and if you come across any other document type that you are using then just add it's extension to the doc type and it should be good.
------------------------------
Srinath MR
------------------------------