Project editor settings are ignored for certain files

I have:

• the global Editor settings set to Tabs Only and continuation to Regular Indent
• a QMake project containing a README.md as `OTHER_FILES` entry
• the project Editor settings set to Spaces Only and continuation to Spaces or Regular Indent (makes no difference). Indentation size is set to 2.
• Pressed TAB in the README.md and got a TAB character instead the expected two spaces.
• Interesting: inside the QMake .pro file, pressing TAB really inserts spaces

So it seems the project settings are valid, but ignored for this file.

BTW: If I change the global Editor settings to Spaces Only, it instantly insert spaces in the README.md also.