The "View Extras" menu will certainly show up for data files if there are sidecars.
I haven't tried the "extras" function with data files, and it could be useful to be able to browse, say, an .xml sidecar written out by calibre, but MC wouldn't be able to search or setup views based on the info in the "extra" sidecars, right? MC doesn't write it's own sidecars for data files, or read metadata from existing sidecars into the tags for data files (unless that's changed?), but it's nice to know one could at least view them.
I think the issue is not as much about being able to see metadata, so much as it is about being able to migrate metadata for data files into MC's database somehow so that one could search, edit the metadata, and create useful view schemes. The user-friendliest way to do that would be to allow for direct import of info from files or from a few standard sidecar formats, or for some kind of database scraping, but I understand that's a significant investment of developer time for a niche interest (and potentially out of scope in some cases).
What seems like it might be a lighter lift (and what I've advocated for previously: see 2) in
https://yabb.jriver.com/interact/index.php/topic,103267.msg717180.html#msg717180) would be to just have an option for MC to write sidecars for data files the same way it does for video files. Once MC writes (and reads) its own sidecars for data files, it would be relatively easy for a user (i.e. me) to script a "conversion" from other software's sidecars to MC sidecars. Presto, instant import of existing metadata.
EDIT: Come to think of it, I wonder what would happen if I just put a jriver formatted sidecar next to a data file, changed it's media type to video and issued an "import from file" command? It's a hack, but for a one-time data migration it might work. A native solution would certainly be better, but it's something to tinker with when I get home ;-)