成员的完整列表，这些成员属于 com.netease.nimlib.sdk.avchat.model.AVChatAttachment,包括所有继承而来的类成员

getDuration()	com.netease.nimlib.sdk.avchat.model.AVChatAttachment
getExtendMessage()	com.netease.nimlib.sdk.avchat.model.AVChatAttachment
getState()	com.netease.nimlib.sdk.avchat.model.AVChatAttachment
getType()	com.netease.nimlib.sdk.avchat.model.AVChatAttachment
isMultiUser()	com.netease.nimlib.sdk.avchat.model.AVChatAttachment
toJson(boolean send)	com.netease.nimlib.sdk.msg.attachment.MsgAttachment