modules/spring-commons/src/main/java/io/restassured/module/spring/commons/HeaderHelper.java
public static String findContentType(Headers headers, List<Object> multiParts, SpecificationConfig config) {
        String requestContentType = headers.getValue(CONTENT_TYPE);
        if (StringUtils.isBlank(requestContentType) && !multiParts.isEmpty()) {
            requestContentType = "multipart/" + config.getMultiPartConfig().defaultSubtype();
        }

        EncoderConfig encoderConfig = config.getEncoderConfig();
        if (requestContentType != null && encoderConfig.shouldAppendDefaultContentCharsetToContentTypeIfUndefined() && !StringUtils.containsIgnoreCase(requestContentType, CHARSET)) {
            // Append default charset to request content type
            requestContentType += "; charset=";
            if (encoderConfig.hasDefaultCharsetForContentType(requestContentType)) {
                requestContentType += encoderConfig.defaultCharsetForContentType(requestContentType);
            } else {
                requestContentType += encoderConfig.defaultContentCharset();
            }
        }
        return requestContentType;
    }
public static String findContentType(Headers headers, List<Object> multiParts, SpecificationConfig config) {
        String requestContentType = headers.getValue(CONTENT_TYPE);
        if (StringUtils.isBlank(requestContentType) && !multiParts.isEmpty()) {
            requestContentType = "multipart/" + config.getMultiPartConfig().defaultSubtype();
        }

        EncoderConfig encoderConfig = config.getEncoderConfig();
        if (requestContentType != null && encoderConfig.shouldAppendDefaultContentCharsetToContentTypeIfUndefined() && !StringUtils.containsIgnoreCase(requestContentType, CHARSET)) {
            // Append default charset to request content type
            requestContentType += "; charset=";
            if (encoderConfig.hasDefaultCharsetForContentType(requestContentType)) {
                requestContentType += encoderConfig.defaultCharsetForContentType(requestContentType);
            } else {
                requestContentType += encoderConfig.defaultContentCharset();
            }
        }
        return requestContentType;
    }
TODO Extract content-type from headers and apply charset if needed!