The “java upload” function in the file manager is not to upload Java, it’s to upload with Java. Specifically, it exposes a Java Applet which could be used to upload files instead of the HTML uploader. If there is any browser which still supports such security disasters.
But InfinityFree provides PHP/MySQL hosting only. Other programming languages, like Java, are not supported. iFastNet’s premium hosting does support more programming languages than free hosting, but Java is not among them either.