How can I get MIME type of an InputStream of a file that is being uploaded?

Question

Simple question: how can I get MIME type (or content type) of an InputStream, without saving file, for a file that a user is uploading to my servlet?

Answer 1

I wrote my own content-type detector for a byte[] because the libraries above weren't suitable or I didn't have access to them. Hopefully this helps someone out.

// retrieve file as byte[]
byte[] b = odHit.retrieve( "" );

// copy top 32 bytes and pass to the guessMimeType(byte[]) funciton
byte[] topOfStream = new byte[32];
System.arraycopy(b, 0, topOfStream, 0, topOfStream.length);
String mimeGuess = guessMimeType(topOfStream);

...

private static String guessMimeType(byte[] topOfStream) {

    String mimeType = null;
    Properties magicmimes = new Properties();
    FileInputStream in = null;

    // Read in the magicmimes.properties file (e.g. of file listed below)
    try {
        in = new FileInputStream( "magicmimes.properties" );
        magicmimes.load(in);
        in.close();
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }

    // loop over each file signature, if a match is found, return mime type
    for ( Enumeration keys = magicmimes.keys(); keys.hasMoreElements(); ) {
        String key = (String) keys.nextElement();
        byte[] sample = new byte[key.length()];
        System.arraycopy(topOfStream, 0, sample, 0, sample.length);
        if( key.equals( new String(sample) )){
            mimeType = magicmimes.getProperty(key);
            System.out.println("Mime Found! "+ mimeType);
            break;
        } else {
            System.out.println("trying "+key+" == "+new String(sample));
        }
    }

    return mimeType;
}

magicmimes.properties file example (not sure these signatures are correct, but they worked for my uses)

# SignatureKey                  content/type
\u0000\u201E\u00f1\u00d9        text/plain
\u0025\u0050\u0044\u0046        application/pdf
%PDF                            application/pdf
\u0042\u004d                    image/bmp
GIF8                            image/gif
\u0047\u0049\u0046\u0038        image/gif
\u0049\u0049\u004D\u004D        image/tiff
\u0089\u0050\u004e\u0047        image/png
\u00ff\u00d8\u00ff\u00e0        image/jpg