The attachment URL will only use one of the preset thumbnail sizes available to it, or the full attachment if none is available. This includes the base thumbnail size that you've chosen, as well as any multiples based on your chosen UHD-related settings.
When the user includes FILE with a specified width, it should automatically pick the next largest thumbnail than that width for displaying inline, using inline style to resize down to the desired width.
There is code in there that tracks the widths that are really requested, with the idea being that at some point we add a way for admins to whitelist some of those values, but no one ever expressed a need for it, so we never created an interface. We can add that to 4.2 since there are a lot of FILE-related updates scheduled for that branch.
I am changing this to a feature request.