Raise error when trying to create parameter that accepts an array of files

[danielhauagge]

Apparently swagger 2.0 can't handle an array of files (see this issue on swagger's API) as a parameter, maybe flask-restplus should raise an error if the user tries to do this.

I ran into this while trying to add an argument with something like this

parser.add_argument('image', type=load_image, location='files',
                    required=True, action='append')

The swagger specification that is generated for that parameter looks like

{
    "collectionFormat": "multi", 
    "description": "", 
    "in": "formData", 
    "items": {
        "type": "file"
    }, 
    "name": "image", 
    "required": true, 
    "type": "array"
},