I would like to set Qt Creator (the one which is in the Nokia Qt SDK, not the one in Canonical PPAs) as the default program to open .pro Qt project files. But it appears that my Ubuntu install (12.04 while I am writing the question) recognizes .pro files as plain text files instead of Qt project files. I know that I could fix the problem by setting Qt Creator as the default program for opening plain text files but I want to keep on opening plain text files with the program I currently use for this (gedit).
So my question is : how can I do for making my Ubuntu install recognizing .pro files as Qt project files instead of plain text files ?
NB : I have already looked at Ubuntu Tweak to associate Qt project files with Qt Creator but I did not find anything relevant (perhaps I missed it too).
EDIT : the solution is in the Edit of Kalle Elmér's answer.