Multipart default improvements, mixed file/in memory streaming decoder #1865
Configuration arguments are good if we avoid case classes, so more arguments (with defaults) can be added later in a compatible way.
I don't know that we care that much about max size per part? If we limit the total size of the body, and we limit the number of parts so we don't get flooded with 1,000,000 single-byte temp files (if the threshold is turned down), then I'm not sure what we're protecting ourselves from. Though I'd take a peak at other common libraries, as we can learn a lot from history of more mature implementations.