Symptoms

When trying to open a jar file to install a plugin in Windows, the following error message appears:

Failed to load Main-Class manifest attribute from <file_path>\plugin_name.jar

Cause

Trying to open the .jar file directly via Windows.

Resolution

To install a plugin into Confluence go inside the UI at Confluence Admin > Find/Install Plugins/Add-Ons (the wording is a bit different depending on which version you're on – just look for the words 'Add-on' or 'Plugin')

This brings you to the Confluence Universal Plugin Manager, which will allow you up to find or upload a plugin as you desire. For more information see Installing Marketplace apps.