In KiCad V5.99 the extension of the project file changed from “[project].pro” to “[project].kicad_pro”.
On my Linux Mint system, there is no mime type for “[project].kicad_pro” and my OS thinks it’s a text file.
If this is just a quirk on my particular system I’ll let it go, but if others ( Linux, Mac, Windoze) are also affected by this, I’ll make a bug report for it.
I use kubuntu. In KDE system preferences, file associations it is possible to assign .kicad_pro to kicad nigthly. As my system is set to Spanish I cannot tell you the exact paths to do it.
In other desktop managers there must be a similar feature.
Manually adjusting file associations should not be necessary and should be a part of the install process.
At the moment I assume this was overlooked when the filename was changed from “.pro” to “.kicad_pro”, and if this is true, it is a bug.