How is the content in the cts variable for the uploadFile function generated?

Example: var cts = "LyoKICogalF1ZXJ5IEphdmFTY3JpcHQgTGlicmFyeSB2MS4zLjIKICogaHR0cD.......

Edit: Found it, Base64 encoding

Yep, Base64. :-) You can use the .NET Framework or even some sites to do the conversion.