     }
 }
 
-/// type of MultipartFormData
-public protocol MultipartFormData {}
-
-/**
-    Helps to encode multipart-form-data's body part from the data.
+/**
+    Protocol to define a MultipartFormData parameter.
+    The implementation must be a class because the value of Target#parameters is AnyObject.
+*/
+public protocol MultipartFormData: class {}
+
+/**
+    Class to define a MultipartFormData parameter from a data.
 
     The body part data will be encoded using the following format:
 

    /**
        - parameter data: The data to encode into the multipart form data.
    */
    init(data: NSData) {
        self.data = data

        self.fileName = nil

}

/**
    Class to define a MultipartFormData parameter from a file.

    The body part data will be encoded using the following format:

