core/src/main/java/org/nanohttpd/protocols/http/response/Response.java
public boolean useGzipWhenAccepted() {
        if (gzipUsage == GzipUsage.DEFAULT)
            return getMimeType() != null && (getMimeType().toLowerCase().contains("text/") || getMimeType().toLowerCase().contains("/json"));
        else
            return gzipUsage == GzipUsage.ALWAYS;
    }
public boolean useGzipWhenAccepted() {
        if (gzipUsage == GzipUsage.DEFAULT)
            return getMimeType() != null && (getMimeType().toLowerCase().contains("text/") || getMimeType().toLowerCase().contains("/json"));
        else
            return gzipUsage == GzipUsage.ALWAYS;
    }
Else decide whether or not to use Gzip.