Element: [Bug Report] Vue v2.4.1 give an error in upload component

[kevinongko]

After updating Vue to 2.4.1 the upload component show error in console when uploading a file.

Element UI version

1.3.7

OS/Browsers version

macOS sierra 10.12.5 / Chrome Version 59.0.3071.115

Vue version

2.4.1

Reproduction Link

https://jsfiddle.net/KevinOngko/pj71jkyw/375/

Steps to reproduce

Upload an image

What is Expected?

no error in console.

What is actually happening?

console show
[Vue warn]: Avoid using non-primitive value as key, use string/number value instead.

[Leopoldthecoder]

Fixed in #5872