I have built a replacement for and excel spreadsheet with filemaker and it is great however there is one BIG problem.

Excel will give a preview of an illustrator file, Filemaker will not, so when a file is inserted in Filemaker there is no image, but in Excel it produces the image, this is not great surely there is a way around this so Filemaker can show the Illustrator file :)

are you trying to store .ai files in container fields with a preview, or add them to a layout as a graphic element?

If you're trying to put them in container fields, you can see a preview if you store as reference rather than storing directly in the field. If you want to see a preview of a picture when stored in the container field, then you'll need a different format. You can save as either .eps or .pdf from illustrator, and both those file types will show up in containers and as layout graphics. Both of these formats will maintain vector editability in illustrator.