Just wanted to bump this as still being a problem after 4.8.
I have tried both of the bif files below, both match the case just in case there was any case sensitivity issues. Both are in the same folder as the .mp4 file and have the same name, just different extensions. The bifs work fine when pulled from a web server.
file://ext1:/Movies/829c5a50-403f-417b- ... 60c71d.bifext1:/Movies/829c5a50-403f-417b-a733-05397560c71d.bif
If anyone has any additional thoughts I welcome them.