我认为对于解码，你可以尝试这样的东西，

String delims="[,]";
    String[] parts = base64ImageString.split(delims);
    String imageString = parts[1];
    byte[] imageByteArray = Base64.decode(imageString );

    InputStream is = new ByteArrayInputStream(imageByteArray);

    //Find out image type
    String mimeType = null;
    String fileExtension = null;
    try {
        mimeType = URLConnection.guessContentTypeFromStream(is); //mimeType is something like "image/jpeg"
        String delimiter="[/]";
        String[] tokens = mimeType.split(delimiter);
        fileExtension = tokens[1];
    } catch (IOException ioException){

    }