In resteasy-client.js do not use JSON.stringify on entity of type FormData

}else if(this.entity instanceof Object){
   if(!contentTypeSet || REST._isJSONMIME(contentTypeSet))
	data = JSON.stringify(this.entity);
}

}else if(this.entity instanceof Object && !(this.entity instanceof FormData)){
 	if(!contentTypeSet || REST._isJSONMIME(contentTypeSet))
 		data = JSON.stringify(this.entity);
}