@janvankuijk. Thanks for posting.
Jan, your own post answers your question, actually. The way Google preview works is that it tells goole’s preview app the web address of the file to be previewed, and Google Preview accesses the file, and displays it in its preview widget. If your files are on an intranet, which does not have a publicly accessible domain name, then Google cannot access it.
Even without Google Preview, it is impossible to find a file that is located on an Intranet via the Internet. Isn’t that right?
So, there is no way that I or anyone can help you. If your intranet domain is publicly available from the Internet, then it will work. If it is not a public domain, there is no way that it can work.
Hope this helps.