Upload files when adding content to group don't work well

Using Group 8.x-1.0-beta2 on Drupal 8.1.7 and get an error when I try to upload file or image with the create content form in a group (group/{gid}/create/content_type). I can upload only 1 file on creation and have to modify entity after saving to add more file (using /node/{nid}/edit/).

It look like an Ajax issue, I get those log in firebug when uploading a file :

NetworkError: 500 Internal Server Error - http://devdruplocal:8010/fr/group/1/node/create/article?element_parents=field_image/widget&ajax_form=1&_wrapper_format=drupal_ajax

AjaxError: 
Une erreur HTTP AJAX s'est produite.
Code de statut HTTP : 500
Informations de débogage ci-dessous.
Chemin : /fr/group/1/node/create/article?element_parents=field_image/widget&ajax_form=1
StatusText: Internal Server Error
ResponseText : {"message":"A fatal error occurred: Serialization of \u0027Symfony\\Component\\HttpFoundation\\File\\UploadedFile\u0027 is not allowed"}