[Bug Report] on-success event in el-upload component fires random on of times, not every time the file is uploaded,

### Element UI version
2.3.6

### OS/Browsers version
Mac/ Chrome

### Vue version
2.5.13

### Reproduction Link
https://jsfiddle.net/65v71hao/3/

### Steps to reproduce
Try Uploading multiple files(More than 4), Check the no of files uploaded.

### What is Expected?
Fire the on-success event everytime a file is uploaded successfully.
And don't omit the files being uploaded.

### What is actually happening?
on-success event is firing random no of times.