"The given path's format is not supported."

Rather than using str_uploadpath + fileName, try using System.IO.Path.Combine instead:

Path.Combine(str_uploadpath, fileName);

which returns a string.

I see that the originator found out that the error occurred when trying to save the filename with an entire path. Actually it's enough to have a ":" in the file name to get this error. If there might be ":" in your file name (for instance if you have a date stamp in your file name) make sure you replace these with something else. I.e:

string fullFileName = fileName.Split('.')[0] + "(" + DateTime.Now.ToString().Replace(':', '-') + ")." + fileName.Split('.')[1];

For me the problem was an invisible to human eye "‪" Left-To-Right Embedding character.
It stuck at the beginning of the string (just before the 'D'), after I copy-pasted the path, from the windows file properties security tab.

var yourJson = System.IO.File.ReadAllText(@"D:\test\json.txt"); // Works
var yourJson = System.IO.File.ReadAllText(@"‪D:\test\json.txt"); // Error

So those, identical at first glance, two lines are actually different.