I have a large amount of .dia in a folder. I can't remember the content. With Dolphin, even under 'Preview', I don't seem to be able to preview the files. So I need Nautilus, which does exactly that. Overall, I pretty much prefer Dolphin, though, and suggest to improve the preview. Reproducible: Always
Thanks for the report! If you want previews for .dia files, we need a volunteer who writes a thumbnailer for this format (I don't think that there is one for KDE).
Resetting assignee to default as per bug #305719