ios上消息附件下载为啥总是失败

ios, v3.1.2 FULLSDK

在环信移动客服上发送一个附件到app客户端, app尝试调用
[[EMClient sharedClient].chatManager asyncDownloadMessageAttachments:model.message progress:nil completion:^(EMMessage *message, EMError *error) { }];
结果回调总是失败,  显示 EMErrorFileDownloadFailed

打开环信log, 其中的url地址在浏览器中可以正常下载, 但是为啥环信sdk下载就是不行?
[2016/5/24 10:20:39:327]: log: level: 1, area: 1, RECV:
{ verison : MSYNC_V1, command : SYNC, payload : { status : { error_code : 0 }, metas : [ { id : 199680070829212248, timestamp : 1464056439427, ns : CHAT, payload : { chattype : CHAT, from : hizhutestservice1, to : a85c854e_1102_c3a8_316d_e6a2c49ddd73, contents : [ { contenttype : FILE, displayname : iPhoneSize.jpg, remotepath : https://kefu.easemob.com/v1/Tenant/15196/MediaFiles/065361a8-c39b-46b9-9535-26972fac7e00aVBob25lU2l6ZS5qcGc=, secretkey : , filelength : 458705 } ], exts : [ { key : weichat, type : 8, value : {"agent":{"avatar":"//kefu-prod-avatar.img-cn-hangzhou.aliyuncs.com/avatar/15196/264c5418-ed30-4b95-bc4b-b68afd76f4a6@0-0-200-200a|300h_300w|.png","userNickname":"Admin测试客服"},"agentUsername":null,"ctrlArgs":null,"ctrlType":null,"event":null,"metadata":null,"originType":null,"queueId":null,"queueName":null,"visitor":{"callback_user":"a85c854e_1102_c3a8_316d_e6a2c49ddd73","companyName":null,"description":null,"email":null,"mp":null,"openid":null,"phone":"13301935205","qq":null,"sex":null,"source":null,"tags":null,"trueName":null,"userNickname":"蘑菇蘑菇蘑","weixin":null}} } ] } } ], next_key : 199680070829212248, queue : admin } }