Some comments:

I second Julian Reschke's proposal to return 201/Created from the POST requests. The Location header should be set to the URL use to test for chunked upload; except for the final POST, which of course returns the URL to the actual binary uploaded.
Request Extension for POST: Yes, we need a request extension to properly assign selectors. Suggest to use res instead of html. We also use this extension in the Default GET Servlet to request streaming the result in case we need an extension.
Chunk numbers: Using chunk numbering as propose implies that there is a predefined size of each chunk. The respective specification is missing in this proposal. Otherwise, instead of using chunk numbers you could use size ranges. For example the discovery request returns the size of contiguous data already uploaded successfully. The POST requests in indicate the file offset (the number of bytes sent is equal to the Content-Length header.
If the check request has to has an extension, this should be JSON to reflect the actual data format expected in the response. This URL should (see above) be used as the Location header on the 201/CREATED responses to the POST request.

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[CONF] Apache Sling > Chunked File Upload Support

Chunked File Upload Support