Hi there!

We have a situation where we use rc-upload in the following manner:

• User selects file on file system
• We use transformFile to encrypt the contents of that file on the browser
• The encryption generates additional metadata that we want to pass as headers
• This metadata is not available until transformFile is completed.

It would be great if headers could also be a function that takes in the result of transformFile and returns the headers.

I tried to make a PR for this but it was not possible bc of permissions.

The changes to do this are pretty quick:
in AjaxUploader.tsx

change

headers: props.headers,

to

headers: typeof props.headers === 'function' ? props.headers(transformedFile) : props.headers,

and also update the README comment to me

|headers| object/function(blob) | {} | http headers to post, available in modern browsers. Can be a function that takes in the result of transformFile |