Sidecar files, also known as buddy files or connected files, are supplementary files that provide extra information about the main file.
In Libiry, we can have two sidecar files per book file: a cover image file and a markdown file with book data in it.
In Libiry, the relationship between the source file and the sidecar file is based on the file name; sidecar files have the same base name as the source file, but with a different extension.