xml文件下载报This XML file does not appear to have any style information associated with it. The document

可能是原文件缺失了文件声明
正常的xml文件

<?xml version='1.0' encoding='UTF-8'?>
<definitions xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:activiti="http://activiti.org/bpmn" xmlns:bpmndi="http://www.omg.org/spec/BPMN/20100524/DI" xmlns:omgdc="http://www.omg.org/spec/DD/20100524/DC" xmlns:omgdi="http://www.omg.org/spec/DD/20100524/DI" typeLanguage="http://www.w3.org/2001/XMLSchema" expressionLanguage="http://www.w3.org/1999/XPath" targetNamespace="http://www.activiti.org/processdef">
 
</definitions>

有问题就像这样

<definitions xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:activiti="http://activiti.org/bpmn" xmlns:bpmndi="http://www.omg.org/spec/BPMN/20100524/DI" xmlns:omgdc="http://www.omg.org/spec/DD/20100524/DC" xmlns:omgdi="http://www.omg.org/spec/DD/20100524/DI" typeLanguage="http://www.w3.org/2001/XMLSchema" expressionLanguage="http://www.w3.org/1999/XPath" targetNamespace="http://www.activiti.org/processdef">
 
</definitions>

没有<?xml version='1.0' encoding='UTF-8'?>

2020-05-29补充

有时候通过接口下载XML文件，即使文件中加了<?xml version='1.0' encoding='UTF-8'?>还是报这样的错，这时候可以在response设置Header

JAVA版

response.setHeader("Content-Disposition", "attachment; filename=" + name + ".bpmn20.xml");
response.setContentType("application/octet-stream");

go语言版

//设置Content-Type
w.Header().Add("Content-Type", "application/octet-stream")
w.Header().Add("Content-Disposition", "attachment; filename=\""+name+"\".bpmn20.xml")