Cannot save multiple file or image references in node content type

I am trying to save multiple files/image references in a content type but the reference has not been saved.
If the field has limit one, the reference is saved ok.
This is the format of the data that I want to save with an example of image and file fields:
{
"type": "property",
"title": "property 1",
"body": {
"und": [
{
"value": "property detail "
}
]
},
"field_documents": {
"und": [
{
"fid": 6,
"display": 1
},
{
"fid": 5,
"display": 1
}
]
},
"field_image": {
"und": [
{
"fid": 5,
"display": 1
}
]
}
}