filename field is missing when uploading a blob

[tangye1234]

I found when uploading a blob, there is no way to append the filename to the form data.

As the standard API say:

FormData.prototype.append(key, file)
FormData.prototype.append(key, blob, filename)

from
https://developer.mozilla.org/zh-CN/docs/Web/API/FormData/append

The filename of blob will be blob when omitting the filename field of append method
Following is the snapshot of the request payload

Some 3rd party upload server decides the filename only by reading the filename field.

So I think the request.js can receive another option field for the purpose of denoting the 3rd argument filename in the FormData.prototype.append