I think the Web browser itself is either adding the trailing slash at the end of URLs, so I do not see a point of blocking it.
And if you block it, why so?
In some cases, the non-trailing slash and trailing slash version don’t redirect to the correct version. This can cause issues with crawling and duplicate content.
I might be wrong to understand the point, but I would recommend you to configure your origin host / server to not show the list of indexes, if worried about that?, either have an empty index.php file inside /uploads/ directory (if not by default).
If using Apache and .htaccesss file, it’s done by adding below line (usually returns the 403 Forbidden error) when someone tries listing the contents of the directory: