As the title indicates, it is impossible to import any file into Kiri:Moto running any recent version of iOS/iPadOS browser (including Safari/Firefox/Chrome/iCabMobile/Puffin, among others). This error occurs under all iOS Versions from 11.x.x to 14.2.x.
This appears to be a bug in the underlying Java code which does not indicate support for stl files.
This limitation does not exist using other browser-based iOS 3D design apps, such as TinkerCAD and AstroPrint.
I have attempted file imports using all current public versions of Kiri:Moto with the same result.
If there is another workaround, I am unable to find it.
Please look into this. If I may be of any further assistance, please let me know.