Bug 492660

Confirmed with Dolphin 24.12.1 on:
Operating System: Solus 4.7
KDE Plasma Version: 6.2.5
KDE Frameworks Version: 6.10.0
Qt Version: 6.8.1
Kernel Version: 6.12.9-312.current (64-bit)
Graphics Platform: Wayland
Processors: 16 × AMD Ryzen 7 PRO 4750U with Radeon Graphics
Memory: 29.1 GiB of RAM
Graphics Processor: AMD Radeon Graphics

rename files which have same suffix `.idx` also makes file lost it's suffix.
seems like the `.idx` cannot be correctly batch renamed.
seems like a Qt bug, or the mime database doesn't contains the `.idx` suffix.
tested with minimal qt demo:
#include <QCoreApplication>
#include <QMimeData>
#include <QMimeDatabase>
#include <QDebug>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    QMimeDatabase db;
    const QString extension = db.suffixForFileName("/home/debian/Downloads/file.idx");
    qDebug() << "######  " << extension; // output an empty string
    return 0;
}

also, this is not a bug of qt, qt reads all mime from system, the idx is not included in, see:

 debian@debian  /usr/share/mime/text  cat x-mpsub.xml                                                                                                ✔  959  16:53:10
<?xml version="1.0" encoding="utf-8"?>
<mime-type xmlns="http://www.freedesktop.org/standards/shared-mime-info" type="text/x-mpsub">
  <!--Created automatically by update-mime-database. DO NOT EDIT!-->
  <comment>MPlayer subtitles</comment>
  <comment xml:lang="uk">субтитри MPlayer</comment>
  <comment xml:lang="sv">MPlayer-undertexter</comment>
  <comment xml:lang="ru">Субтитры MPlayer</comment>
  <comment xml:lang="pl">Napisy MPlayer</comment>
  <comment xml:lang="es">subtítulos de MPlayer</comment>
  <comment xml:lang="de">MPlayer-Untertitel</comment>
  <sub-class-of type="text/plain"/>
  <glob pattern="*.sub"/>
</mime-type>

so you can install the mimetype description file or create a file yourself to solve this issue.

(In reply to xust from comment #3)
> also, this is not a bug of qt, qt reads all mime from system, the idx is not
> included in, see:
> 
>  debian@debian  /usr/share/mime/text  cat x-mpsub.xml                     
>  ✔  959  16:53:10
> <?xml version="1.0" encoding="utf-8"?>
> <mime-type xmlns="http://www.freedesktop.org/standards/shared-mime-info"
> type="text/x-mpsub">
>   <!--Created automatically by update-mime-database. DO NOT EDIT!-->
>   <comment>MPlayer subtitles</comment>
>   <comment xml:lang="uk">субтитри MPlayer</comment>
>   <comment xml:lang="sv">MPlayer-undertexter</comment>
>   <comment xml:lang="ru">Субтитры MPlayer</comment>
>   <comment xml:lang="pl">Napisy MPlayer</comment>
>   <comment xml:lang="es">subtítulos de MPlayer</comment>
>   <comment xml:lang="de">MPlayer-Untertitel</comment>
>   <sub-class-of type="text/plain"/>
>   <glob pattern="*.sub"/>
> </mime-type>
> 
> so you can install the mimetype description file or create a file yourself
> to solve this issue.

but I think kio could do some workaround? such as if suffix obtained from qmimedatabase is empty, do split by dot, and use the last part as suffix? only if `fullName.split('.').count() >=2`

Well that raises 2 questions:

1. Should this rely on MIME-types at all or simply see everything after the last dot as the file-extension (possibly with a switch?)
2. What's the upstream of those MIME-type files so the extension can be added there? Is it https://gitlab.freedesktop.org/xdg/shared-mime-info/-/releases ?

(In reply to Tarulia from comment #5)
> Well that raises 2 questions:
> 
> 1. Should this rely on MIME-types at all or simply see everything after the
> last dot as the file-extension (possibly with a switch?)
> 2. What's the upstream of those MIME-type files so the extension can be
> added there? Is it
> https://gitlab.freedesktop.org/xdg/shared-mime-info/-/releases ?

1. I have the same idea with you, as I suggeted to KIO in my comments, and I'll make a pr see if KIO accept.
2. I'm not clear about this.

A possibly relevant merge request was started @ https://invent.kde.org/frameworks/kio/-/merge_requests/1846

(In reply to xust from comment #2)
> rename files which have same suffix `.idx` also makes file lost it's suffix.
> seems like the `.idx` cannot be correctly batch renamed.
> seems like a Qt bug, or the mime database doesn't contains the `.idx` suffix.

Qt and KDE software use standard shared mime database https://gitlab.freedesktop.org/xdg/shared-mime-info
If an extension needs to be recoginsed, that's where it needs to be fixed.