Files such as ISO or IMG and folder structures such as VIDEO_TS or BDMV are generally referred to as "disk image" formats because they're basically "cloning" the structure and content from a DVD or Blu-Ray optical disc.
Such formats are not supported in Plex.
Simply put, these formats just don't fit into the idea behind the Plex ecosystem. They hide content behind built-in menu systems. Not only would the menus add extra UI that a user would need to wade through to get to the content, but there also simply isn't any good way that they could be presented in the myriad different Plex apps that are available - from iOS to Android, from the web browser to the Roku. It just isn't feasible to try and present a DVD menu type interface in some of the devices.
For television content, disk image formats will lump multiple episodes on a single disk. That makes it much more difficult to try and allow a user to directly access the particular episode they may want to watch. No one would want to have to fast-forward through episodes 10 and 11 if all they wanted was to watch episode 12 simply because they couldn't access 12 directly.
Since these disk image formats aren't supported, we recommend that users convert such content into more common and compatible media formats. There are many formats and tools available. Some can retain full quality of the original video or others could greatly compress the filesize with nearly no noticeable visual (or audio) quality loss.