In Windows file explorer, navigate to the folder where the VTT file is located. Make sure it is not saved with a txt extension which can happen when vtt files are editied in notepad. So look for “YourFileName.vtt.txt”. This would keep our Browse For file window from seeing the vtt file, as its actually a txt file. You might have to modify your windows explorer options to turn on “Show File name extensions”.
If that checks out okay, in the Browse for File window, expand the file names column to see the actual extensions of the files. The windows “Type” could be reporting the vtt as a xml due to a misconfiguration, but we can’t tell in the current screen shot waht the real file names are.