webapi保存接口附件管理保存报错

客户端保存同样大小的附件是没问题，但是通过webapi则报错json长度问题，请问怎么处理？
具体报错信息：

[tr][td]response_error:{"InnerExWrapper":{"IsEmpty":false,"Data":{},"HelpLink":null,"Message":"使用 JSON JavaScriptSerializer 进行序列化或反序列化时出错。字符串的长度超过了为 maxJsonLength 属性设置的值。\r\n参数名: input","Source":null,"StackTrace":" 在 System.Web.Script.Serialization.JavaScriptSerializer.Deserialize[T](String input)\r\n 在 Kingdee.BOS.JSON.JSONArray..ctor(String parameters)\r\n 在 Kingdee.BOS.ServiceFacade.KDServiceFx.RequestExtractor.GetServiceParameters(String[] paras)\r\n 在 Kingdee.BOS.ServiceFacade.KDServiceFx.ExecuteServiceModule.OnProcess(KDServiceContext context)\r\n 在 Kingdee.BOS.ServiceFacade.KDServiceFx.ModulePipeline.ExcuteRequest(KDServiceContext context)\r\n 在 Kingdee.BOS.ServiceFacade.KDServiceFx.RequestExcuteRuntime.BeginRquest(RequestExtractor requestExtractor, WebContext webCtx)\r\n 在 Kingdee.BOS.ServiceFacade.KDServiceFx.RequestExcuteRuntime.StartRequest(RequestExtractor requestExtractor, WebContext context)\r\n 在 Kingdee.BOS.ServiceFacade.KDServiceFx.KDSVCHandler.ExecuteRequest(WebContext ctx, RequestExtractor requestExtractor)\r\n 在 Kingdee.BOS.ServiceFacade.KDServiceFx.KDSVCHandler.ProcessRequestInternal(WebContext webCtx, RequestExtractor requestExtractor)","TargetSite":null,"ExceptionType":"System.ArgumentException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089","InnerException":null},"ClassName":"Kingdee.BOS.ServiceFacade.ServiceException","Message":"Unknown Error","Data":null,"InnerException":null,"HelpURL":null,"StackTraceString":null,"RemoteStackTraceString":null,"RemoteStackIndex":0,"ExceptionMethod":null,"HResult":-2147467259,"Source":null,"WatsonBuckets":null,"_httpCode":500}