I'm trying to arrange for "Insert File" to open in a particular folder (cross platform).  It seems it will only go to a specific file that I fully specify.  If the file doesn't exist, it seems to go to some other place (like the last folder or the documents folder).  Any way to control this?  Thanks

I don't think so, but you could probably do it with a plug-in. The next version of FileMaker may have this feature according to the just-released roadmap:

https://community.filemaker.com/community/platform/product-roadmap

File-based script steps — Create scripts that read, write, and manage external data files to support external logging and data export in a custom format. Replaces need for many file-based plug-ins.