If your goal is to upload a different file at each user iteration then the best is to copy your files into the <project_folder>\custom-resources folder.

Then in your POST definition you can use ${NL-CustomResources}\${myfile.filename} syntax.

${NL-CustomResources} variable will automatically be replaced by NeoLoad to an absolute path that exists on any load generator whatever the operating system. Your files into custom-resources folder will be sent to all your load generators.