If you use drag and drop into a Qt wasm app (e.g. from Windows Explorer into the app), and the file is small (like 1 to 5kb), the file in /qt/tmp/ has no bytes in it at all. This appears to be true on both Chrome and Firefox. 

/qt/tmp does gets the content of the file but size shows zero. If the dropped file is larger like 100kb or large then it shows the correct size.

Just drop any file in attached example and see the browser log for size.