Support for image metadata

[pegler]

Is your feature request related to a problem? Please describe.
Services like Unsplash require the image be attributed to the photographer. There doesn't seem to be any way to pass additional metadata about the selected image from the service to the uploader.

Describe the solution you'd like
It would be great if services could pass a metadata object along with the file blob into the next() call. I don't personally have a need for effects to be able to read or modify the metadata, but passing the metadata into effects may make sense. Finally the metadata would need to be passed into the uploader so it can be sent to the server.

Describe alternatives you've considered
I tried setting custom attributes on the file blob, but because the file is manipulated by effects as well as run through safeBlobToFile, blobToUpploadFile, and safeUpploadFileToFile, those attributes get lost.

[phlegx]

@pegler did you found a solution? I have the same problem.