Accessing the original file path in the user’s computer is not possible, either through JavaScript or PHP’s $_POST, because it would pose an invasion to the user’s privacy.
You can only access the file’s name and a few other properties after it already has been uploaded, using the $_FILES superglobal on your PHP script.
You then have to complete the upload process by moving/renaming the uploaded file (which is stored in a temporary folder) into your chosen directory, otherwise it will be automatically deleted by PHP at the end of the script.
For more:
PHP Manual on File Uploads
You can also use a built-in WordPress function:
wp_handle_upload()
