Deve estar no diretório abaixo:
/usr/share/applications/mimeapps.list
Se não existir, você pode criá-lo e colocar essa linha neste arquivo:
sudo nano /usr/share/applications/mimeapps.list
Você também pode criar este arquivo em seu diretório pessoal:
nano ~/.local/share/applications/mimeapps.list
E coloque esta linha no arquivo:
application/vnd.nokia.qt.qmakeprofile=qtcreator.desktop
Por exemplo, meu conteúdo do arquivo mimeapps.list é:
$ cat /usr/share/applications/mimeapps.list
[Default Applications]
application/wps-office.et=/usr/share//applications/wps-office-et.desktop
application/wps-office.ett=/usr/share//applications/wps-office-et.desktop
application/wps-office.xls=/usr/share//applications/wps-office-et.desktop
application/wps-office.xlt=/usr/share//applications/wps-office-et.desktop
application/vnd.ms-excel=/usr/share//applications/wps-office-et.desktop
application/msexcel=/usr/share//applications/wps-office-et.desktop
application/wps-office.xlsx=/usr/share//applications/wps-office-et.desktop
application/wps-office.xltx=/usr/share//applications/wps-office-et.desktop
vnd.ms-excel.sheet.macroenabled.12=/usr/share//applications/wps-office-et.desktop
vnd.ms-excel.template.macroenabled.12=/usr/share//applications/wps-office-et.desktop
application/wps-office.dps=/usr/share//applications/wps-office-wpp.desktop
application/wps-office.dpt=/usr/share//applications/wps-office-wpp.desktop
application/wps-office.ppt=/usr/share//applications/wps-office-wpp.desktop
application/wps-office.pot=/usr/share//applications/wps-office-wpp.desktop
application/vnd.ms-powerpoint=/usr/share//applications/wps-office-wpp.desktop
application/vnd.mspowerpoint=/usr/share//applications/wps-office-wpp.desktop
application/powerpoint=/usr/share//applications/wps-office-wpp.desktop
application/wps-office.pptx=/usr/share//applications/wps-office-wpp.desktop
application/wps-office.potx=/usr/share//applications/wps-office-wpp.desktop
vnd.ms-powerpoint.presentation.macroenabled.12=/usr/share//applications/wps-office-wpp.desktop
vnd.ms-powerpoint.template.macroenabled.12=/usr/share//applications/wps-office-wpp.desktop
application/wps-office.wps=/usr/share//applications/wps-office-wps.desktop
application/wps-office.wpt=/usr/share//applications/wps-office-wps.desktop
application/wps-office.doc=/usr/share//applications/wps-office-wps.desktop
application/wps-office.dot=/usr/share//applications/wps-office-wps.desktop
application/vnd.ms-word=/usr/share//applications/wps-office-wps.desktop
application/msword=/usr/share//applications/wps-office-wps.desktop
application/msword-template=/usr/share//applications/wps-office-wps.desktop
wps-office-doc.png=/usr/share//applications/wps-office-wps.desktop
wps-office-dot.png=/usr/share//applications/wps-office-wps.desktop
Para mais informações sobre os aplicativos padrão do tipo MIME, consulte este link no Arch linux Wiki .