Add an "Ebooks" category in file filters to recognize EPUB, MOBI, and other eBook formats

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Is your feature request related to a problem? Please describe.

Currently, Nextcloud does not recognize EPUB files (or other eBook formats like MOBI, FB2, etc.) as a specific file type in the file listing filters. Instead, EPUB files appear as "unknown" or simply as regular files.

This limitation makes it difficult to filter and manage eBooks in Nextcloud, especially for users who store large digital libraries. Right now, users can only filter by "Documents," but EPUBs and other eBooks do not appear in this category.

Describe the solution you'd like

It would be great to introduce a new "Ebooks" category in the file filter section, alongside existing categories like Documents, PDFs, Audio, and Video.

This new category should automatically include file types like:

• application/epub+zip (EPUB)
• application/x-mobipocket-ebook (MOBI)
• application/fb2+xml (FB2)
• application/vnd.comicbook+zip (CBZ)
• application/vnd.comicbook-rar (CBR)

Describe alternatives you've considered

1. Manually tagging files – While Nextcloud allows users to tag files, it does not provide an easy way to filter by MIME type, making this an impractical workaround for large collections.
2. Using third-party apps – Some apps (like EPUB viewers) allow opening eBooks, but they do not modify the way files are categorized in Nextcloud's UI.
3. Editing the database manually – Modifying the MIME type database could work temporarily, but this is not a long-term or sustainable solution, as it could break with updates.

Additional context

Adding an "Ebooks" category would significantly improve file organization for users managing digital libraries, online reading rooms, or personal book collections within Nextcloud.

A potential enhancement could also be to allow users to customize file categories in the settings, so they can define their own file groups based on MIME types.

The issue names no files, tests, or entry points. Start by locating the file-filter category definitions and MIME-type mappings, then trace the existing Documents, PDFs, Audio, and Video filters; done means EPUB, MOBI, FB2, CBZ, and CBR files appear under a working Ebooks category with suitable coverage.