Ability to upload files by clicking external button

[shibby]

Hi @charliekassel

Thanks for this awesome job. Super clean&easy code. I would like to request a feature.
For single file uploads, when a file selected it immediately starts uploading file. Instead of this, i want to manage my own upload button. I'm creating a pull request for this but my changes are not working as expected. I will write some comments into it too.

Thanks.