EpubCCEPUBCheck PKG-016: uppercase .EPUB extension
Updated:
EPUBCheck PKG-016 is a warning message: “Use only lowercase characters for the EPUB file extension for maximum compatibility.” The file extension is written in uppercase (.EPUB); lowercase .epub is the most compatible. The content of the book is not affected, only how some apps and stores recognise the file.
The official message
Use only lowercase characters for the EPUB file extension for maximum compatibility.
EPUBCheck's suggestion: For maximum compatibility, use ".epub".
- Default severity
- Warning. The EPUB is still valid, but something is likely to cause problems in some reading apps. EPUBCheck exits with 0 unless you pass --failonwarnings.
- EPUBCheck version
- 5.4.0
Typical causes
- A file saved or renamed on a system or tool that uppercased the extension.
- A file name copied from an old archive or a download system that uses uppercase names.
How to fix it
- Rename the file so it ends in .epub, in lowercase.
- Nothing inside the EPUB needs to change; validate the renamed file if you want a clean report.
- If a script produces the file, fix the extension in the script so the next build is right.
When it's not this
If the extension is something other than .epub (.zip, .epub3), the message is PKG-017.
Terms used here
Other PKG messages
PKG-017Uncommon EPUB file extension.PKG-014The EPUB contains empty directory "…".PKG-018The EPUB file could not be found.PKG-012The file name "…" contains non-ASCII characters, which might create interoperability issues with older reading systems.PKG-020OPF file "…" could not be found.PKG-011The file name "…" is not allowed to end with ".".
Sources
Message text and severity from EPUBCheck 5.4.0 (W3C, BSD-3-Clause). Explanation by EpubCC.