Got a Dataobject with many_many attachments and have set the appropriate has_many relationships (I think so?)

class VideoSubmission extends DataObject {

private static $many_many = array(
    'Attachments' => 'File'
  );

public function getCMSFields() {
    $fields = parent::getCMSFields();
    $fileField = UploadField::create('Attachments', 'Upload some  files');
   return $fields;
}

// Add a data extension for file
class SubmissionFileExtension extends DataExtension{
    private static $belongs_many_many = array(
        'VideoSubmissions' => 'VideoSubmission'
    );
}
File::add_extension('SubmissionFileExtension');

This works. But if I replace the fileUploadField above with:

$fileField = FileAttachmentField::create('Attachments', 'Upload some  files')
      ->setView('grid');

Then files will upload but not attach.

I'm able to see the files added with UploadField (previously), I can detach them but can't add anything new.
I'm on the most recent composer installed ss (today).

My composer.json

{
	"name": "silverstripe/installer",
	"description": "The SilverStripe Framework Installer",
	"require": {
		"php": ">=5.3.2",
		"silverstripe/cms": "3.1.13",
		"silverstripe/framework": "3.1.13",
		"silverstripe-themes/simple": "*",
		"silverstripe/memberprofiles": "dev-master",
		"rywa/silverstripe-foundation-forms": "~1.0@dev",
		"burnbright/silverstripe-externalurlfield": "*@stable",
		"silverstripe-australia/gridfieldextensions": "dev-master",
		"unclecheese/dropzone": "dev-master"
	},
	"require-dev": {
		"phpunit/PHPUnit": "~3.7@stable"
	},
	"config": {
		"process-timeout": 600
	},
	"minimum-stability": "dev"
}

Looks right. Do you need the file extension? I've confirmed that this works:

class TestFilePage extends Page {

	private static $many_many = array (
		'Files' => 'File'
	);


	public function getCMSFields() {
		$f = parent::getCMSFields();
		$f->addFieldToTab('Root.Upload',FileAttachmentField::create('Files'));

		return $f;
	}
}

Also, `add_extension()` is kind of old school. You'll want to move that to a definition in the config yaml.

Hm... I'm not attaching to a page, I'm trying to attach to a DataObject and the files won't upload in the CMS (the frontend behaves the same).

What I do:
- drag in the files (I see the files appear in the field, ready to load)
- the progress bars remain at 0 even if I set ->setAutoProcessQueue to true or false. I noticed that when false, clicking the Upload Files button doesn't trigger the upload either. I wonder if it's because I'm on MAMP? (at the same time, UploadField does work)
E.g.
https://www.dropbox.com/s/fm8jj6z88deyvmt/Screenshot%202015-05-29%2009.51.37.png?dl=0

What I did notice, in the network, I'm getting an "undefined" / cancelled GET request right after I add files into the field:
https://www.dropbox.com/s/tayikbws833zbe2/Screenshot%202015-05-29%2009.57.29.png?dl=0

Something's up for sure...

Here's my whole class, if you want to test it:

<?php

class VideoSubmission extends DataObject {

  private static $db = array(
    'Title' => 'Varchar(255)',
    'Text' => 'Text',
    'City' => 'Varchar(255)',
    'State' => 'Varchar(20)',
    'ZipCode' => 'Varchar(20)'
  );

  private static $has_one = array(
    'Member' => 'Member'
  );

  private static $many_many = array(
    'Files' => 'File'
  );

  public function getCMSFields() {
    $fields = parent::getCMSFields();
    $fileField = FileAttachmentField::create('Files', 'Upload some files')
      ->setView('grid');
      ->setAutoProcessQueue(true) // do not upload files until user clicks an upload button
      ->setAcceptedFiles(array('.jpg', '.jpeg', '.pdf','.doc','.docx', '.mov', '.mp4', '.gp4', '.avi', '.mp3', '.wav'));
    $fields->addFieldsToTab("Root.Main",
      array(
        new TextField("Title", "Titulo"),
        new TextareaField("Text","Descripcion"),
        new TextField("City", "Ciudad"),
        new DropdownField("State", _t("MEMBERS.State","Estado"), Helpers::getStateDropDown()),
        new TextField("ZipCode", "Codigo Postal"),
        $fileField
      )
    );
    return $fields;
  }

}

So you're managing this in ModelAdmin, then? I can confirm that this works:

class TestFileDataObject extends DataObject {

	private static $many_many = array (
		'Files' => 'File'
	);


	public function getCMSFields() {
		$fields = FieldList::create(TabSet::create('Root'));
		$fields->addFieldToTab('Root.Main', FileAttachmentField::create('Files'));

		return $fields;
	}
}

class TestAdmin extends ModelAdmin {

	private static $managed_models = array (
		'TestFileDataObject'
	);

	private static $menu_title = 'Test';

	private static $url_segment = 'test';
}

The "undefined" path is obviously the issue. Can you inspect the field and check the data-config setting? It should look something like this:

data-config="{"url":"admin\/test\/TestFileDataObject\/EditForm\/field\/TestFileDataObject\/item\/1\/ItemEditForm\/field\/Files\/upload","maxFilesize":2,"uploadMultiple":true,"clickable":".dropzone-select","params":{"SecurityID":"6688d3e8e524ba62a22fe1db0c184ac40c7d64e8"},"thumbnailsDir":"dropzone\/images\/file-icons\/64px","thumbnailWidth":64,"thumbnailHeight":64,"urlSelectDialog":"admin\/test\/TestFileDataObject\/EditForm\/field\/TestFileDataObject\/item\/1\/ItemEditForm\/field\/Files\/select"}"

I assume your "url" setting contains no value?

Nope, the config is good and the url is there all clean. For the heck of it, I used your test class and it's also failing to upload files which I thought was really weird. I removed some composer dependencies to see if any clashed, I went from 3.1.13 to 3.1.14.x-dev to check it out. No changes.

Config looks good:

{"url":"admin\/test\/TestFileDataObject\/EditForm\/field\/TestFileDataObject\/item\/new\/ItemEditForm\/field\/Files\/upload","maxFilesize":32,"uploadMultiple":true,"clickable":".dropzone-select","params":{"SecurityID":"51137d84be772294c952ae9f8407e5829294fa70"},"thumbnailsDir":"dropzone\/images\/file-icons\/64px","thumbnailWidth":64,"thumbnailHeight":64,"urlSelectDialog":"admin\/test\/TestFileDataObject\/EditForm\/field\/TestFileDataObject\/item\/new\/ItemEditForm\/field\/Files\/select"}

I actually thought it may have to do with a DataObject's getURLPrefix() but all this looks okay to me (as much as I can understand :)

The only clue I have so far is this: I noticed the growl-y error in CMS when creating a new TestFileDataObject in ModelAdmin and clicking "Create" after dragging in some files. The log states:

[30-May-2015 04:10:56] Warning at framework/model/UnsavedRelationList.php line 148: Invalid argument supplied for foreach() (http://itestigo.dev/admin/submissions/VideoSubmission/EditForm/field/VideoSubmission/item/new/ItemEditForm

Once a TestFileDataObject has an ID, the errors disappear but the behavior remains (no way to upload & attach / attaching from file library works).

The method from UnsavedRelationList.php is:

/**
* Add a number of items to the relation.
* 
* @param array $items Items to add, as either DataObjects or IDs.
* @return DataList
*/
public function addMany($items) {
	foreach($items as $item) {
		$this->add($item);
	}
	return $this;
}

$items (from wherever they are passed to this) are null at the point (caught it to see), hence I guess the error.

I had another person report this, but I still can't replicate. Might want to follow along here and add your input. https://github.com/unclecheese/silverstripe-dropzone

This issue should be resolved by the latest commit: https://github.com/unclecheese/silverstripe-dropzone/commit/c3f5129e08b74d8e0d5060b66b0ba2792a13bf3f