Sending a File Message.

FileMessage.java

//File Message
// Sending a file message with a raw file
List<FileMessage.ThumbnailSize> thumbnailSizes = new ArrayList<>(); 
thumbnailSizes.add(new ThumbnailSize(100,100));

FileMessageParams params = new FileMessageParams()
        .setFile(FILE)             
        .setFileName(FILE_NAME)
        .setFileSize(FILE_SIZE)
        .setMimeType(MIME_TYPE)
        .setThumbnailSizes(thumbnailSizes)
        .setCustomType(CUSTOM_TYPE)
        .setData(DATA)     
        .setMetaArrayKeys(metaArrayKeys)

groupChannel.sendFileMessage(params, new BaseChannel.SendFileMessageHandler() {
    @Override
    public void onSent(FileMessage fileMessage, SendBirdException e) {
        if (e != null) {    // Error.
            return;
        }
    }
});