After my ride my 510 beeped and shutdown when I hit the stop button. I hooked it up to my computer to upload the file and I got a message that the file type was not recognized. I then took that same file and tried to upload it to Strava and it worked. Any idea why it worked for one but not the other?

They're programmed differently. Maybe different versions of the API. When you uploaded it the first time, some kind of exception was thrown.

You can probably export a gpx from Strava and upload that to your other platform (Garmin?).

A GPX export can't be imported as an Activity into Strava because it doesn't contain your time, gps, HR, speed, and Power Meter data like a .FIT file does.

GPX is a route file that your bike computer uses for navigation while riding. That same bike computer will simultaneously create and write to a .FIT or .TCX file that is then uploaded to Strava when the ride is complete.

But why does Garmin not like this .fit and Strava does?