As the hubby of a dedicated quilter/sewer, I can certainly say that there are varying levels of hold strength with Velcro. In fact, I can remember using some on an electronic project years ago that, once it was engaged we had to use a pry bar to get the two pieces apart. There are also lots of different sizes of the barbs and loops to equate with differing applications. You can find a whole wall of Velcro products at Home Depot or Lowe's or, if there is a sewing supply/fabric store nearby, they should be able to help, too. You should see light, medium and heavy hold strengths.
Having said all that, I'm reminded of someone's earlier comment about not using Velcro for fear that the extinguisher could become a projectile in a panic stop. very true, and that's why I anchored mine with sheet metal screws.
So, Todd: Use big sheet metal screws, not Velcro, on an extinguisher. When you drill the holes for the screws, dip the drill into Butcher's Floor Wax (Johnson's works well, too) - The paste kind....comes in a can about 5" in diameter. Dip the drill in about an inch deep - that will prevent the carpet fibers from attaching to the drill. Once the holes are drilled, dip the screws into the same wax and screw them in. BTW: this is how professional audio installers quickly drill holes in cars, too.
Drill at slow speed if it makes you feel better, but it's not necessary...The carpet should stay put.