FIX - use ImageMagick to determine size of svg images (#12230)

SVG files can have dimensions expressed in inches, centimeters, etc., which may lead to the dimensions being misinterpreted (e.g. “8in” ends up as 8 pixels).

If the file type is `svg`, ask ImageMagick to work out what size the SVG file should be rendered on screen.

NOTE: The `pencil.svg` file was obtained from https://freesvg.org/1534028868, which has placed the file in to the public domain.

spec/lib/upload_creator_spec.rb

@@ -535,6 +535,20 @@ RSpec.describe UploadCreator do
     end
   end
 
+  describe "svg sizing" do
+    let(:svg_filename) { "pencil.svg" }
+    let(:svg_file) { file_from_fixtures(svg_filename) }
+
+    it "should handle units in width and height" do
+      upload = UploadCreator.new(svg_file, svg_filename,
+        force_optimize: true,
+      ).create_for(user.id)
+
+      expect(upload.width).to be > 100
+      expect(upload.height).to be > 100
+    end
+  end
+
   describe '#should_downsize?' do
     context "GIF image" do
       let(:gif_file) { file_from_fixtures("animated.gif") }