I saw a similar posting on the FM forum and realized I have the exact same problem: I've never gotten INSERT FROM URL to work with "FILE" so I think I'm doing something wrong.
In my case, I have a simple INSERT FROM URL with this for the URL portion:
"file://" & Get(DesktopPath) & "myimage.jpg"
I get "Connection failed."
As the other poster wrote, I've also tried various combinations of "file:", but with no success. I could have missed it, but I could not find an actual INSERT FROM URL example in the FMP 19 help with "file".
This example is a regular FMP sample application, but I also read that the file protocol is not supported in WebDirect. Is that true or did I misunderstand that?
There's a cool utility on the Mac (probably others on Windows) called PathFinder that, when you right click a file, it gives you several ways to get a path....including "URL".
What I often do when my first attempt at freehanding in an URL fails: I insert the file in question, as a reference, into a handy global container field. Then I invoke Data Viewer and ask to see GetAsText (that field) and compare that to what I was attempting to use as the URL