# EPUBCheck PKG-017: uncommon file extension, how to fix it

Source: https://epubcc.com/epubcheck/pkg-017/ (updated 2026-11-04)

EPUBCheck PKG-017 is a warning message: “Uncommon EPUB file extension.” The file does not end in .epub, so some reading apps and stores may not recognise it. EPUBCheck reports the same text, "Uncommon EPUB file extension", as PKG-017 (a warning) and as PKG-024 (a usage message); the cause and the fix are the same.

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

## Typical causes

- A file saved with another extension, such as .zip or .epub3, or with none.
- A double extension added by a download or email client, such as book.epub.zip.
- PKG-024: the same message at usage level, shown when EPUBCheck runs with -u (--usage).

## How to fix it

1. Rename the file to end in .epub.
2. Nothing inside the EPUB needs to change: the content, the mimetype file and the package document stay as they are.
3. If a script or service produces the file, fix the extension there so the next build is right.

## When it's not this

If the extension is .epub but in uppercase, the message is PKG-016.

## Sources

- EPUBCheck 5.4.0 message texts (MessageBundle.properties, w3c/epubcheck on GitHub): https://github.com/w3c/epubcheck/blob/v5.4.0/src/main/resources/com/adobe/epubcheck/messages/MessageBundle.properties (read 2026-11-04)

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