In the top class of documents, icons are specialized to give a better idea of what type of a document, to separate message from movie, track from text. This is especially complicated when it comes to graphics, because it is often crucial for what we do with them when it comes to distinguishing between different vector and bitmap formats.
Only then, when we know which animal species we are dealing with, we come to content and QuickLook thumbnails. Of course, the small preview we get in a thumbnail image can give us valuable traces of the type of document we see, but can also be thoroughly misleading. What we need to know is whether this document is MyLastHoliday.jpeg, MyLastHoliday.pdf, or MyLastHoliday.mov. While QuickLook superimposes a Play icon on the latter, which will tell us it's a movie, the first two can be presented with the same thumbnails.
QuickLook also doesn't know what we see. Some first pages of PDF files and other files may look very pretty, but be of limited value in revealing exactly which of a dozen similar-looking documents they might be. When a thumbnail is not easy to read, and may be of several different types of documents, the effort used to generate it has been wasted.
To distinguish between different types of documents, we also need a file type recognition system. How this is achieved is always controversial, but some system is prone to errors. Managing such errors and remaining informative to the user is crucial, and in this case, macOS is currently short.
A potential solution is simply to display a generic document icon. Providing the least amount of information to the user, they only tell which top class class of object they are looking at.
For some types of documents, macOS becomes a bit more specific, and displays an icon generic for that type, here a QuickTime movie. The experienced user will recognize it as a sign of trouble, knowing that a thumbnail should be displayed if everything is in order with that file. However, there is nothing inherent to this icon that spells problems, and it is easy to believe that the file is a movie, as perhaps all that happened, the extension was inadvertently changed from .mob, something trampled over its OST type, or the server returned wrong MIME type.
For other document types, such as JPEG images, this is the current answer from the built-in QuickLook generator: It sits and spins forever. This is too unclear. The user does not know if macOS could not find the right thumbnail generator, whether it is a macOS error, an error file, or an error.
Even in the less cossetted environment of the command line, we expect commands that come up with an error and some will return a code or message to inform us why our desire may not be satisfied on this occasion. The current "solution" in QuickLook is like the annoying friend who, when asking a question, just doesn't answer. If they do not know the answer to your question, it is the answer to the question, not the spin for eternity.
When QuickLook thinks that it cannot provide a suitable thumbnail for a document, all that has to do is substitute a generic document icon defaced with a question mark to indicate its confusion. It contains more valuable meaning that only thinks about what to say, but never says anything. These are basic semiotics.