Alright perhaps you would need to experiment a bit further but here's a hint from Kapsel colleagues - you could apply to your own use cases with UI5:
As you know batch media is supported, but there is no specific Kapsel API for this. It is up to UI5, datajs, or whatever JavaScript library is being used to create the batch request.
You would likely need to use the FileReader API to convert the File object to raw binary or base64 when creating the request.
There was a testing of the FileReader to convert to base64 and passed it through datajs to create the batch request. Note that request.body needs to be used instead of request.data with datajs to prevent the request handlers from trying to convert the data as it only supports JSON, XML, and text.