I don’t know since when it is happening.
I download a iso image (for instance system rescue CD) and within dolphin I click on it and it open it with kwrite (my default text app)
If I right click on the iso, the context menu shows k3b as the first option. If in the context menu, if I select “open with-other” it says "remember application for all files of type “plain text document” (text/plain) but going to system-settings-applications-file associations in text/plain there is no .iso anywhere
And the .iso extension in application/x-cd-image is associated to k3b
From the applications you name I assume you are using KDE.
InKDE System Settings there is Personal Settings > Applications (i hope it is called like that, I have the Dutch translations here). There is File associations. Check there about what is configured about the .iso suffix.
I haven’t personally used KDE and Dolphin in a long time, but IIRC,
Within Dolphin,
Rt-click on the ISO file, select Properties.
There used to be just a link and then later a button that says something like “File Associations”
Click on that and inspect what you see… My guess this is where you have a mis-configuration listing it as text-plain and opened by default with kwrite.
There is a small possibility that you accidentally downloaded a text file instead of an .iso file. Happens to me sometimes that I get a html-page instead of a .zip or a .pdf file. Try
file --mime /location/of/the/iso_file
in a terminal window (e.g. konsole) and post the output here (between CODE] and /CODE] tags).