How to show the author as entity reference of a file used in a node?
Yes, ultimately a query needs the file ID. There are a few ways you can get the file ID considered. You should pick the mode most consistent with the rest of your site build:


If you are using the Panels suite you then have a ton of options both within the "Views content panes" module (that comes as a part of CTools) and within Panels itself for setting up the passing of arguments. My main recommendation under this approach would be to use the "context" concept within panels to go from the node object to the file object that can then be passed to a contextual filter.
You've tried Entity Views Attach. This works well if it is appropriate to show the author of the file within the View Mode of the file.
You've also tried the "Provide default value" option under contextual filter handing but there isn't a suboption for "value from field X on current node". These are "default argument handlers" and here is the code for the one you are using: http://cgit.drupalcode.org/views/tree/modules/node/views_handler_argument_node_nid.inc and the generic API page for them: https://api.drupal.org/api/views/plugins!views_plugin_argument_default.inc/group/views_argument_default_plugins/7 If you are comfortable with PHP you can write one custom to your use case.
Views Field View (https://www.drupal.org/project/views_field_view) is maybe the ugliest (but perhaps the easiest) option. It would allow you to construct a View that takes the argument of the NID (which you already have working) and using that to get the FID as field which would then be passed to another View. A View inside a View.