Is there a way to reliably disable mipmapping in the file itself, so that blenderplayer will respect the setting and not interpolate textures?
And no, I'm not making a Minecraft clone.
Mipmaps is a system setting, it would be set by the users startup blend file. Not sure that anyone would recommend overriding a users choice.
To change it you would use python to set bpy.context.user_preferences.system.use_mipmaps
use_interpolation
for a texture but it may be ignored in the game engine.
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This is related to mipmap settings, you can change in user preferences > systems > mipmap
But this only work inside blender.
In game (standalone) you need a script.
from bge import render
render.setMipmapping(0)
related: Python API