How to bind to the uploader to pass custom parameters to FormData

After a bit more trial and console.log fun, we have found a solution that works, but it's not pretty.

In the media.js file under MediaButton > events

// Refresh content when frame opens
this._frame.on( 'open', function() {
   // adding the custom parameter to the uploader
   var some_custom_param = this.controller.get('some_custom_param');
   if(some_custom_param){
      this._frame.uploader.uploader.uploader.settings.multipart_params.some_custom_param = some_custom_param;
   }
   var frameContent = this._frame.content.get();
   if ( frameContent && frameContent.collection ) {
      frameContent.collection.mirroring._hasMore = true;
      frameContent.collection.more();
   }
}, this );

this is just a test. next steps would be:
- get feedback on if there is a more elegant way to add the multipart_params
- create our own field type 'file_more_advanced' so we don't modify metabox's core files
- OR collaborate on extending the official file_advance field logic to support appending the multipart_params

Thoughts?

Hello Peter.
the real case is an advanced plugin that handles handles the uploading, storing and managing access for sensitive data. If you share your github handle, I would be happy to give you access to the repo that explains everything in detail, including a demo-plugin we created as a proof of concept.

We ended up creating a custom field type called secure_file_advanced that allows the passing of custom parameters to the $_REQUEST before the file is uploaded. To do this we needed to get very intimate with how metabox.io has implemented the WordPress AJAX Media Uploader on the back and frontend (three uploaders deep). Our case may be an edge case, but it dose extend the flexibility of existing file_advanced field.

Regarding field type file: no for all the reasons that we went with file_advanced in the first place.
Also, adding the upload_dir attribute to the field meta, visible on the frontend, is not desired.
Also, with thousands of members we needed more than just a single upload-directory.