Tika Fails to detect some EML files if extension is not .eml

I have an eml file that can be detected as message/rfc822 only if the file extension is .eml,  otherwise it will be detected as text/plain.  Following is the code that I use to detect the file type and extension.

       TikaConfig config = TikaConfigFactory.getTikaConfig();

       Detector detector = config.getDetector();

       Metadata metadata = new Metadata();

       TikaInputStream stream = TikaInputStream.get(fis = new FileInputStream(filePath));

       metadata.add(Metadata.RESOURCE_NAME_KEY, filePath);

       MediaType mediaType = detector.detect(stream, metadata);

       MimeType mimeType = config.getMimeRepository().forName(mediaType.toString());

       String tikaExtension = mimeType.getExtension();

When the sample file has .eml extension,  mimeType is message/rfc822 and  tikaExtension is eml. When I change the extension to .txt, mimeType is text/plain and  tikaExtension is .txt.

The same mimeType and tikaExtension should be detected regardless the file extension.