# EPUBCheck PKG-024: Uncommon EPUB file extension

Source: https://epublab.com/epubcheck/pkg-024/ (updated 2026-11-03)

EPUBCheck PKG-024 is a usage message: “Uncommon EPUB file extension.” The file does not end in .epub, so some reading apps and stores may not recognise it.

- Official message (EPUBCheck 5.4.0): Uncommon EPUB file extension.
- Default severity: Usage
- EPUBCheck's suggestion: For maximum compatibility, use ".epub".

## Typical causes

- A file saved with another extension, or a double extension added by a download or email client.

## How to fix it

- Rename the file to end in .epub. Nothing inside the EPUB needs to change.

Message text and severity: EPUBCheck (W3C, BSD-3-Clause). Explanation: EPUBLab.