I read How to upload / attach file using Appium | Selenium Easy in order to understand how people test uploading files in Mobile. In this article, when you started uploading, you want to choose a file from “Files” folder on the Mobile device.
I think, you must do a preprocessing: you have to somehow transfer the file from your Windows’ C:\Users\UserName\Desktop\TEST.pdf into the Mobile device’s "Files" folder. Only after you have prepared the fixture file, you can test uploading the file from the Mobile devices’ "Files" folder.
How to prepare the file on the Movile devices’ “Files” folder? ---- I don’t know. Possibly you want to bundle the fixture PDF file in the APK file?