[Bug Report] (1.2.9) the upload component can't successively select the same file

[shawtung]

Element UI version

1.2.9

OS/Browsers version

macOS 10.12 / Chrome 57.0.2987.133 (64-bit)

Vue version

2.2.6

Reproduction Link

https://jsfiddle.net/shawtung/2xrxjxp4/

Steps to reproduce

多次上传同一个文件。后续选择以及上传无效。
Upload the same file more than once. The first is OK but the following doesn't has any response.

What is Expected?

1.2.8版本会清空input的fileList，所以会正常上传同一个文件多次。
v1.2.8 will clear the fileList of the input element, so that you can select & upload the same file several times, as expected.

https://jsfiddle.net/shawtung/myo5kks2/

What is actually happening?

1.2.9版本后续选择无效，不会触发任何事件。
Invalid following selection and upload in v1.2.9, nothing will happen.

[element-bot]

为了让更多的人能够理解你的问题，同时给遇到相同问题的人提供更好的参考，请尽量使用英语提交你的 issue。相关信息：#4396。

[element-bot]

Translation of this issue:

Element UI version

1.2.9

OS/Browsers version

MacOS 10.12 / Chrome 57.0.2987.133 (64-bit)

Vue version

2.2.6

Reproduction Link

https://jsfiddle.net/shawtung/2xrxjxp4/

Steps to reproduce

Upload multiple files. Subsequent selection and upload invalid.

What is Expected?

1.2.8 version will empty the input fileList, so it will be the same file upload multiple times.

Https://jsfiddle.net/shawtung/myo5kks2/

What is actually happening?

1.2.9 version subsequent selection is invalid, will not trigger any event.