That completely failed to answer my actual question and just reiterated information that I already knew.

At least half of those file formats are more likely to be harmful than a .msg file, so I really don't believe that the restriction is for security reasons, but is more likely to just be another basic QOL feature that Intuit isn't willing to pay a dev to implement.  Honestly, given how many basic, obvious feature requests have literally years of forum posts asking for, I have to wonder if Intuit even has any devs working on QBO at all.