Version: unspecified (using Devel)
There is 'Preview' action already, unfortunately it uses internal previewer which is very simple and in many cases broken (like for .diff files - shows partial diff, barely useful).
'Open with' action would be very useful, esepcially if it resembled its functional equivalent from Dolphin.
With regard to the specific case of diff files, it depends on the KPart associated to this mimetype: you're most likely using KDiff. If you use KatePart, the diff file should be shown in its entirety.
*** This bug has been marked as a duplicate of bug 179066 ***