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Imagine I have family or default activity and will open all image file in Gwenview. Imagine that I'm working on an open source program and I'm graphic designer, so I need to open each graphic file from directory of this project in Gimp.

Solution is to makes mime types category settings configurable per activity / directory.

I think the idea of per-activity file associations has some merit even for the single-user use case. Another scenario is a picture project which involves manipulating files using a few different tools between initial creation, alterations and transformations, touch-up, etc. You could have one activity dedicated to Krita, another for Gimp, another for Inkscape, and then the default for other activities is to just use Gwenview. Other scenarios could include the developer who wants to open things in different IDEs depending on the activity, or the writer who uses different writing tools for different stages.

Another example:
We are teacher and having three activities:
C, Python and Presentation

Imagine we prepared beginning in C presentation on C activity, beginning in Python presentation on Python activity. In next step we link both documents to Presentation activity. On presentation activity we have prevents energy saving option enabled.