I have tried just about every trick in the book. Setting fill="none" does not work on "path" tags either. Removing the "stroke" color manually allows Filemaker to change the line color of the icons at least, but it will still fill paths.
I have been working with icons8.com to find a simple solution. They have adjusted their "simple svgs" in black, so they come out as "path" with no mention of fill. The color and fill of those icons work in Filemaker. The scaling is another problem. Filemaker does not respect the given 50x50 viewBox and scales to its own 24x24 viewBox, making it impossible to scale the icons.
So out of luck here, I guess.