FileStream could not find a part of the path

Using FileStream with Blazor results in Could not find a part of the path error while trying to load file. I tried using Path.Combine instead of direct path, but it didn't help

FileStream stream = new FileStream("Content/Testing/testfile.jpg", FileMode.Open, FileAccess.Read);

If it's relevant, I'm trying to integrate Spine Skeletal Animation C#/MonoGame runtime with KNI MonoGame extension. Everything works fine except one atlas file that is loaded via FileStream and it's hard for me to figure out where exactly lays the issue.