Consolidate FileUpload value by using a single top-level dict

[jtpio]

Fixes part of #2721.
Continuation of #2666.

The FileUpload value now follows this structure:

uploader.value
#=> [
#=>   {
#=>     'name': 'example.txt',
#=>     'type': 'text/plain',
#=>     'size': 36,
#=>     'lastModified': 1578647380733, 
#=>     'content': <memory at 0x10c1b37c8>
#=>   }
#=> ]

[jasongrout]

Thanks!

[pbugnion]

Nice!