media/initiate-upload-with-checksum

Generate a presigned S3 URL for direct upload, with a client-supplied integrity checksum. $checksumSha256 must be the base64-encoded SHA-256 digest the client computed from the file itself, before calling this method - we never compute or invent a value on the client's behalf, since we don't have the file. That exact value is signed into the presigned URL; Amazon S3 rejects the upload outright if the bytes it actually receives don't match it.