That's because the path in OctoPrint is relative to the container root, so you need to go looking for the location the container is mounted to the filesystem.
I do not want to upload to the folder. I want to automate a download of the converted file created by ArcWelder, so I can verify that the print will look as expected, before starting the print
There is also a console version of the arcwelder pre-processor here. You can find the latest release binaries here, though I'm not 100% sure all of the OS builds are working 100%. It can be added to most slicers pretty easilly. What OS are you using to slice?
FYI, I'll be pushing another update soon to handle an issue caused by offsetting X and Y via a g92 command.