I can provide specific command-line steps or links to tools for your exact situation.
Access a version of Eaglercraft (like 1.5.2 or EaglercraftX 1.8) via your browser. epk minecraft file
Have you found a specific EPK file that won't open? Let us know in the comments below! I can provide specific command-line steps or links
You might be implementing as a custom archive reader: epk minecraft file
Because EPK files are designed specifically for the browser-based environment, they are handled differently than standard Minecraft files: