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The reason why an epub's cover is not replaceable will be displayed in the log. It will usually be one of the following three messages:
No supported meta tag or non-xml cover
A cover must have a <meta name="cover" content="xxx/> defined in the opf manifest. Even if this tag is defined, the manifest item that it is linked to must be an image as defined by the media-type of the manifest item. It cannot be an xml file as some svg covers are.
DRM encrypted cover
If an epub has DRM encryption, and the encryption.xml file indicates that the encryption also applies to the cover image, then it cannot be replaced.
Invalid cover image extension
Looking at the filename extension of the image file itself, does it end in .jpg, .jpeg or .png, as these are the only image formats calibre's logic supports replacing.
In the case of the "Check non-replaceable covers", failing any of these conditions means the ePub will appear in the results.
In the case of "Check replaceable covers", it must pass all of these conditions to appear in the results.
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