As suggested by kampfader, the webif log window should definitely output something. I just tried it on my installation and this is what I get if I mis-spell the filename
2019-05-09 18:21:15.744 iptv: unable to open file '/storage/.config/rascal.m4u': No such file or directory
When I succesfully load a file the output in webif log window is something like this
2019-05-09 18:21:49.605 iptv: m3u parse: 176 new mux(es) in network 'test' (total 176)
The file path format is “file:///storage/.config/rascal.m3u”. You are adding it as IPTV Automatic Network?