// than the same number of values for different keys (many map entries), but
		// using a consistent per-value cost for overhead is simpler.
		maxMemoryBytes := 2 * int64(10<<20)
		maxMemoryBytes -= int64(len(name))
		maxMemoryBytes -= mapEntryOverhead
		if maxMemoryBytes < 0 {
			// We can't actually take this path, since nextPart would already have
			// rejected the MIME headers for being too large. Check anyway.