Same problem as described here:
https://pkp.sfu.ca/support/forum/viewtopic.php?f=8&t=14090
I’ve confirmed this on my local install, and I believe on one of our hosted installs as well. Basically, when I upload an HTML file as an issue galley, it’s identified as “Untitled”; then it’s downloaded instead of viewed inline from the TOC. I’m using a very simple HTML file, nothing that doesn’t work as an article HTML galley.