ePub: Cover support for ePub files generated by older versions of Sigil

Describe your suggested feature

Bit of a strange request, but I've been looking at an issue with epub files and their cover not coming through. I think I've narrowed this down to how the Sigil software creates the epub file and the metadata in the content.opf file.

I have a book generated by the older version of Sigil which has the following metadata (summarised)

<?xml version="1.0" encoding="utf-8"?>
<package version="3.0" unique-identifier="pub-id" xmlns="http://www.idpf.org/2007/opf" xml:lang="en">
  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:opf="http://www.idpf.org/2007/opf">
    <dc:title>The Troubles of Miss Nicola the Exorcist: Volume 1</dc:title>
    <dc:creator id="creator01">Ito Iino</dc:creator>
    <meta refines="#creator01" scheme="marc:relators" property="role">aut</meta>
    <meta refines="#creator01" property="display-seq">1</meta>
    <meta refines="#creator01" property="file-as">IINO, ITO</meta>
    <dc:creator id="creator02">Kinokohime</dc:creator>
    <meta refines="#creator02" scheme="marc:relators" property="role">ill</meta>
    <meta refines="#creator02" property="display-seq">2</meta>
    <meta refines="#creator02" property="file-as">KINOKOHIME</meta>
    <dc:creator id="creator03">Joshua Douglass-Molloy</dc:creator>
    <meta refines="#creator03" scheme="marc:relators" property="role">trl</meta>
    <meta refines="#creator03" property="display-seq">3</meta>
    <meta refines="#creator03" property="file-as">DOUGLASS-MOLLOY, JOSHUA</meta>
    <dc:creator id="creator04">Mario Mendez</dc:creator>
    <meta refines="#creator04" scheme="marc:relators" property="role">edt</meta>
    <meta refines="#creator04" property="display-seq">4</meta>
    <meta refines="#creator04" property="file-as">MENDEZ, MARIO</meta>
    <dc:publisher>J-Novel Club</dc:publisher>
    <dc:rights>Copyright © 2023 Ito Iino, Kinokohime</dc:rights>
    <dc:date>2023-08-04</dc:date>
    <dc:language>en</dc:language>
    <dc:identifier id="pub-id">9781718375376</dc:identifier>
    <meta refines="#pub-id" scheme="onix:codelist5" property="identifier-type">15</meta>
    <meta property="dcterms:modified">2023-10-31T03:10:02Z</meta>
    <meta name="Sigil version" content="0.9.14" />
  </metadata>

Then if I compare that to generated by a newer version of Sigil, this has an extra meta name for "cover":

  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:opf="http://www.idpf.org/2007/opf">
    <dc:title>The Troubles of Miss Nicola the Exorcist: Volume 2</dc:title>
    <dc:creator id="creator01">Ito Iino</dc:creator>
    <meta property="role" refines="#creator01" scheme="marc:relators">aut</meta>
    <meta property="display-seq" refines="#creator01">1</meta>
    <meta property="file-as" refines="#creator01">IINO, ITO</meta>
    <dc:creator id="creator02">Kinokohime</dc:creator>
    <meta property="role" refines="#creator02" scheme="marc:relators">ill</meta>
    <meta property="display-seq" refines="#creator02">2</meta>
    <meta property="file-as" refines="#creator02">KINOKOHIME</meta>
    <dc:creator id="creator03">Joshua Douglass-Molloy</dc:creator>
    <meta property="role" refines="#creator03" scheme="marc:relators">trl</meta>
    <meta property="display-seq" refines="#creator03">3</meta>
    <meta property="file-as" refines="#creator03">DOUGLASS-MOLLOY, JOSHUA</meta>
    <dc:creator id="creator04">Mario Mendez</dc:creator>
    <meta property="role" refines="#creator04" scheme="marc:relators">edt</meta>
    <meta property="display-seq" refines="#creator04">4</meta>
    <meta property="file-as" refines="#creator04">MENDEZ, MARIO</meta>
    <dc:publisher>J-Novel Club</dc:publisher>
    <dc:rights>Copyright © 2023 Ito Iino, Kinokohime</dc:rights>
    <dc:date>2023-11-02</dc:date>
    <dc:language>en</dc:language>
    <dc:identifier id="pub-id">9781718375390</dc:identifier>
    <meta property="identifier-type" refines="#pub-id" scheme="onix:codelist5">15</meta>
    <meta property="dcterms:modified">2024-04-08T07:30:34Z</meta>
    <meta content="2.1.0" name="Sigil version"/>
    <meta name="cover" content="Cover.jpg"/>
  </metadata>

This works absolutely fine, Komga loads it perfectly fine.

If I manually edit the older Sigil made file and simply add in the <meta name="cover" content="Cover.jpg"/> line and analyse the file once more, this gets read perfectly.

I was therefore thinking either:

Maybe test for a file in the ePub file named "cover.jpg", I'm not sure if this is universal however.
In the section there's a line <item id="cover" href="Text/cover.xhtml" media-type="application/xhtml+xml"/>, it's probably way more complicated but the cover could be picked up from there by reading the <img alt="Cover" class="cover" src="../Images/Cover.jpg"/> section.
Have a file named book name.cover.extension in the same directory as the epub, that could then be imported at the same time? E.g. "BookName.epub" and "BookName.cover.jpg"

Other details

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Acknowledgements

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gotson / komga