Mehrere bearbeiten und überschreiben, Auswahl wird nicht übernommen / geändert

[Ainschy]

Checklist before I submit this issue report

I confirm that:

• I have tested this with the latest version available
• I have read documentation @ http://metamodels.readthedocs.org/en/ or http://metamodels.readthedocs.org/de/
• I have checked the Contao community forums for references https://community.contao.org/
• I have checked existing issues for duplicates and found none @ https://github.com/MetaModels/attribute_file/issues?q=is%3Aissue

My environment is:

(Please fill in the actual values from your environment)

Key	Value	Comments
PHP version:	7.1.19
Contao version:	4.4.23
MetaModels version:	2.1
Installation via composer:	yes
Installed MetaModels packages:

            "metamodels/core": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_alias": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_checkbox": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_combinedvalues": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_decimal": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_file": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_langcode": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_levensthein": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_longtext": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_numeric": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_select": "dev-feature/contao4 as 2.0.0",
	"metamodels/attribute_tabletext": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_tags": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_text": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_url": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_translatedalias": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_translatedcombinedvalues": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_translatedlongtext": "dev-feature/contao4 as 2.1.0",
	"metamodels/attribute_translatedurl": "dev-feature/contao4 as 2.1.0",
	"metamodels/filter_checkbox": "dev-feature/contao4 as 2.1.0",
	"metamodels/filter_select": "dev-feature/contao4 as 2.1.0",
	"metamodels/filter_tags": "dev-feature/contao4 as 2.1.0",
	"metamodels/filter_text": "dev-feature/contao4 as 2.1.0",
	"metamodels/isotope-bridge": "dev-master as 1.0",
	"metamodels/attribute_isotopeprice": "dev-master as 1.0",
	"metamodels/attribute_isotopebaseprice": "dev-master as 1.0"

Issue description

Die Option Mehrere Bearbeiten / Überschreiben übernehmen die Aufwahl über den Picker nicht in das Eingabefeld und damit nicht in den Datensatz,

Speichern und Neu, erzeugt einen Datensatz, in dem die File Auswahl ebenfalls nicht übernommen wird.

Steps to reproduce

In einen MM mit File attribute Mehrere Bearbeiten aufrufen
Die betreffenden Datensätze auswählen
Dann Bearbeiten / Überschreiben wählen
Die Dateiauswahl über das File Attibute aufrufen
Auswahl / Abwahl wird nicht ans File Field übernommen.
Console zeigt folgenden 500er Fehler:
mootools.min.js:1 POST https://TEST.de/contao?do=metamodel_mm_ballontyp&&act=select&select=edit&rt=X_Px4n9lI-M3JtYtjyf9W_1-euQLgdiqzD9Zs2z14vQ&ref=4mBDdY3O 500 (Internal Server Error)