Remove advice on HTML file extension

[dauwhe]

I thought we had discussed this, but I can't find an issue.

Content Docs says

The XHTML Content Document filename SHOULD use the file extension .xhtml

I think we should delete this sentence, and remove the corresponding test from EPUBCheck. I think this annoys users without helping anyone else. And if you use file extensions to tell you anything about a file, you're going to have a bad time.

[mattgarrish]

Maybe we should do this across the board. We also recommend file extensions for the package document, svg, pls lexicons and smil files.

[dauwhe]

Maybe we should do this across the board. We also recommend file extensions for the package document, svg, pls lexicons and smil files.

Yes!

[mattgarrish]

Ya, I wanted to remove these while retooling the conformance sections, but didn't want to get bogged down in a conformance change. I'll open a PR.

[mattgarrish]

Proposed Solution

Remove the recommended file extensions for Package Documents, XHTML Content Documents, PLS Lexicons and Media Overlay Documents.

Add a note in the package document definition section to refer to the media type registration for more info about file properties.

[dauwhe]

Did some testing. Made the "Ivan nightmare" file where an XHTML content document used the file extension .svg. The resulting EPUB gives a warning in EPUBCheck, of course, but works in:

1. Apple Books
2. Calibre
3. Adobe Digital Editions 3.0
4. Thorium

Interestingly, when I tried to convert the EPUB to a non-EPUB format, the conversion tool complained.

[mattgarrish]

Epubcheck doesn't inspect the content to verify the media type, so it's the easiest to fool. Faking media types was a periodic suggestion in the idpf forums for avoiding fallback requirements.