"Incompatible magic value" error when adding an external Applet within {html} tags on Confluence

I have a page with 3 .JAR attachments. I am trying to execute the applet using this simple code:
{html}
<APPLET CODEBASE="http://XXX:12500/download/attachments/7341517/" ARCHIVE="main.jar,controlP5.jar,core.jar" CODE="main.class" WIDTH=400 HEIGHT=200> </APPLET>
{html}
The page displays correctly, but the applet does not run. I get the following error:
Java.lang.ClassFormatError: Incompatible magic value 1008813135 in class file main
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(Unknown Source)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at sun.plugin2.applet.Applet2ClassLoader.findClass(Unknown Source)
at sun.plugin2.applet.Plugin2ClassLoader.loadClass0(Unknown Source)
at sun.plugin2.applet.Plugin2ClassLoader.loadClass(Unknown Source)
at sun.plugin2.applet.Plugin2ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.plugin2.applet.Plugin2ClassLoader.loadCode(Unknown Source)
at sun.plugin2.applet.Plugin2Manager.createApplet(Unknown Source)
at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Exception: java.lang.ClassFormatError: Incompatible magic value 1008813135 in class file main
Does anyone have any idea what is wrong? Thank you.

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