[KevinH]

Windows (all versions) uses a case insensitive but case preserving file system. Linux is case sensitive and MacOS can easily be either depending on how you format the drive.

So on Windows when Sigil asks for "cover.jpg" the file name returned by the OS could be any of the following as all these names map to the exact same file:

cover.jpg
Cover.jpg
COVER.JPG
CoVeR.jPG
...
etc.

The spec for interpreting a url that resolves to a file path is that it must match in a case sensitive manner: "cover.jpg" only matches to "cover.jpg" and no other variations.

So Windows sees Cover.jpg as the same file as cover.jpg but this is specific to Windows and may not happen in most standalone e-reading devices that are Linux based.

Again, epubcheck checks for mismatches like this to alert you to fix it.

Many developers hate case insensitive file systems and try to avoid them to prevent inadvertant overwriting of files. In my opinion, Windows still being case-insensitive is a real point of aggravation, and something Windows 11 should have finally fixed, but did not. If you ever format a drive in Windows, there must be a way to create a real case-sensitive file system. It is really easy to do on a Mac.

I use a case sensitive file system on my Mac and the "cover.jpg" in your epub does not appear in Preview since it does not exist. It also would break under other e-readers. So stick to case sensitive file names and never use a filename that only differs by case (upper vs lower) of some of the letters, if you want your epub to work everyplace.