Can confirm the issue, affects both webrtc and legacy connection equally with 2.48 Companion
//myfile.txt fails
/myfile.txt works
/Download/myfile.txt works
/AppInventor/assets/myfile.txt works !!
I also tried using Taifun's file component block ".FileListFromAssets", putting the output in a list, then selecting file from a spinner
This fails as it only generates the filename, you have to join the /AppInventor/assets/ path for it to work