基于jQuery的Ajaxs使用FormData上传文件要注意两个参数的设定
processData设为false
把processData设为false，让jquery不要对formData做处理，如果processData不设置为false，jquery会把formData转换为字符串。
contentType设为false
http发送multipart/form-data请求报文示例
POST /api/feed/ HTTP/1.1Accept-Encoding: gzipContent-Length: 225873Content-Type: multipart/form-data; boundary=OCqxMF6-JxtxoMDHmoG5W5eY9MGRsTBpHost: www.myhost.comConnection: Keep-Alive--OCqxMF6-JxtxoMDHmoG5W5eY9MGRsTBpContent-Disposition: form-data; name="lng"Content-Type: text/plain; charset=UTF-8Content-Transfer-Encoding: 8bit116.361545--OCqxMF6-JxtxoMDHmoG5W5eY9MGRsTBpContent-Disposition: form-data; name="lat"Content-Type: text/plain; charset=UTF-8Content-Transfer-Encoding: 8bit39.979006--OCqxMF6-JxtxoMDHmoG5W5eY9MGRsTBpContent-Disposition: form-data; name="images"; filename="/storage/wbavrx.jpg"Content-Type: application/octet-streamContent-Transfer-Encoding: binary这里是图片的二进制数据--OCqxMF6-JxtxoMDHmoG5W5eY9MGRsTBp--
注意Content-Type: multipart/form-data; boundary=OCqxMF6-JxtxoMDHmoG5W5eY9MGRsTBp ，参数boundary为请求参数之间的界限标识。
如果jquery请求设置了contentType，那么就会覆盖了formData的content-type,导致服务器在分隔参数和文件内容时是找不到boundary，报no multipart boundary was found错误
默认情况下jquery会把contentType设置为application/x-www-form-urlencoded。要jquery不设置contentType,则需要把contentType设置为false。
var formData = new FormData($("#uploadform")[0]);$.ajax({ ???url: actionUrl, ???type: ‘POST‘, ???data: formData, ???async: false, ???cache: false, ???contentType: false, ???processData: false, ???...});
Ajax使用formdata异步上传文件，报错the request was rejected because no multipart boundary was found
原文地址：https://www.cnblogs.com/zhangym118/p/9029269.html