If we select multiple pictures on Dolphin you can open them by:

1- right clicking and choosing "Open with"
2- pressing Return

The behaviour is different but should be the same.

I have Gwenview as the default image browser.

Use case 1) will result in a single instance of Gwenview beeing opened and populated with the given list of images. This is correct because the gwenview desktop file as %U in the Exec line, which means it accepts multiple arguments.

Use case 2) will result in as many instances of Gwenview as the number of selected pictures. This is wrong because it ignores the capabilities declared in the desktop file (%U).

Use case 2) makes it easy to accidentally crash a PC or a Terminal Server. Imagine that you are browsing 4G of pictures from a camera and select them all in order to copy them to a local folder. Then press Return by accident. This is a serious issue that is present on many default KDE setups.

The return Key should have Dolphin open all files but instead of processing them in a row and calling each default filetype handler one by one, Dolphin should group them by mimetype and for each mimetype open either a single instance of the handler of multiple instances of the handler depending on the handler's capability, as present in the .desktop file.

Reproducible: Always

Steps to Reproduce:
1. associate a viewer that supports multiple arguments to an image mimetpye (ex: Gwenview or Gimp)
2. select multiple pictures of that mimetype on Dolphin
3. press Return
Actual Results:  
Multiple instances of the viewer are launched.

Expected Results:  
A single viewer should have been launched.

This bug is loosely related to:

https://bugs.kde.org/show_bug.cgi?id=222186