Web reader (and possibly android apps) fail to open books with spaces in chapter file names

Summary

When you have a book which has chapters as

• Some Title split_1.htm
• Some Title split_2.htm etc

The web reader fails to open the book.

https://discord.com/channels/1300864558545764352/1463235071791337547 https://discord.com/channels/1300864558545764352/1461698497538428958

Environment

• 2.6+
• Web reader
• n/a
• n/a

Steps to reproduce

1. Create epub
2. Edit one of the titles of a chapter to add a space
3. Try to open that chapter in the web reader
4. See something like

What is the current bug behavior?

Fails to show chapter

What is the expected correct behavior? (optional)

Should show chapter

Relevant logs and/or screenshots (optional)

Docker logs

[2026-01-20 08:19:43.343 +0000] ERROR: Resource not found in book: Phillip K. Dick - UBIK_split_000.htm
[2026-01-20 08:19:43.359 +0000] ERROR: Resource not found in book: Phillip K. Dick - UBIK_split_002.htm

Additional context (optional)

The problem is that Readium does not seem to be able to deal with this. Will file an issue. See https://github.com/readium/cli/issues/94