Provide the option for upload file by Signed URL

[twkevinzhang]

This is the current popular cloud development approach, and we should support it on both the full-ends.

[n1crack]

Yes we should, I've implemented it before for a different project. but my hands are full atm, I need some time.

[n1crack]

With 4.0, we can customize the uploader. I think, this is possible now.