[GranitStateColin]

Is there an option in Calibre for when it's converting to EPUB 3 to have it name the content (e.g., index_split_xxxx.html) files with an XHTML extension instead of an HTML extension?

As background, the reason I need EPUB 3 vs just continuing to use EPUB 2 is that EPUB 3 supports some important features in highly structured books that EPUB 2 does not. Nested lists and outlines where individual items may span multiple paragraphs (so counter doesn't update on consecutive paragraphs) are common examples, among others. These are standard features in text books and other nonfiction content. If converting from MS Word, which supports these features without doing anything special, when converting in Calibre, they break in EPUB 2, but work perfectly in EPUB 3.

Functionally, I realize there is no difference between the two file extensions, but the EPUB 3 validation tools used by many distribution companies report the HTML file names as a problem (triggers a warning, not an error, but that's enough that some will block the EPUB). I'm trying to automatically output files without having to go into the editor and manually rename all the content files from index_split_xxx.html to index_split_xxx.xhtml).

The validators seem fine with .HTML or .XHTML in EPUB 2, but only .XHTML in EPUB 3.

If there is no built-in way to do this in Calibre, should I submit this as a feature request, or is there perhaps a tool I could run on the final EPUB file after it's created that could go through and rename those files inside the EPUB?