Is there a hook or sth after file is uploaded via file field?
hook_node_insert() and/or hook_node_update() would probably be the right place to do this. Your code might look something like this:

function MYMODULE_node_insert($node) {
  // See if a file has been added
  if ($file_items = field_get_items('node', $node, 'field_file')) {
    // Get the first file item
    $file_item = array_shift($file_items);

    // Load the file object from the cache/database
    $file = file_load($file_item['fid']);

    // The path to the file is in $file-&gt;uri
    do_something_with($file-&gt;uri);
  }
}