If it were filename, you couldn’t have the same filename twice unless they were from different folders on your device/computer. It wouldn’t matter anyway, because you can have multiple observations in the same upload having the same taxa as ID…
I would imagine exif tags could be read in the same way to pick up a taxa. Even if it just attempted to read/find them, and if it can make a match put it in as the taxa, observer can always edit them from there anyway. I don’t name my files either by filename or by exif, but there are plenty of people that do, so this would help a lot of people to reduce their upload workload. It might even encourage some of us to start doing it, and would be excellent for those that prefer to ID themselves before uploading. You could take your time off-line and add the name to the files, so for times when you have no internet access it can mean you could be adding the names to the photos so that the upload goes quicker when you are back online.