CURRENT SITUATION: 1. Right click on a audio file -> "Open with..." and see that "mpv Media Player" is installed. 2. Would like to make this the default application. 3. Choose "Other..." from the menu. 4. The "Open with" dialog opens. 5. Enter "mpv" in the text input field. => "mpv" is shown as valid suggestion. The user might think that this is the correct choice but it is not: If you search the tree view _by hand_ and click "mpv Media Player" then the correct command is put to the text input field ("mpv --profile=pseudo-gui -- %U"). See also https://bugs.kde.org/show_bug.cgi?id=195214 SUGGESTION: Typing text into the input field also searches the tree view below and highlights matches (and hides non-matching tree items). This does not mitigate all possible problems of this dialog but would be a step towards better usability.
Thanks for the report. The "Open with" dialog is part of KIO -> reassigning.
See also this VDG post https://forum.kde.org/viewtopic.php?f=285&t=131014
Like many people, myself included, you thought that text field was a search field to look for applications. In fact, it's actually a place to enter command-line tools to handle the document! This isn't very intuitive, and it will be fixed with Bug 359233. *** This bug has been marked as a duplicate of bug 359233 ***