I'm guessing what you did was at some point you chose Krita as the app for opening FBX files. You can change that by rightclicking the file, choosing properties and then clicking "change" next to the "Opens with:". All in all that shouldn't matter if you're importing the file directly into unity by using its import feature/dragging and dropping. I recommend you look up a tutorial on YouTube for VRChat/Unity that shows how to put clothing (like shirts, jackets, etc.) onto a VRChat avatar when the clothes come as an FBX model, not a UnityPackage. Make sure the tutorial also explains how to properly link or assign the bones (armature) so the clothing moves correctly with the avatar.