FIX: Better error handling if a file cannot be sent

If for some reason `Discourse.store.path_for` returns `nil`, the
forum would throw an error rather than returning 404.

Why would it be `nil`? One cause could be changing the type of
file store and having the `url` field no longer be relative.

app/controllers/uploads_controller.rb

@@ -75,7 +75,11 @@ class UploadsController < ApplicationController
         }
         opts[:disposition]   = "inline" if params[:inline]
         opts[:disposition] ||= "attachment" unless FileHelper.is_supported_image?(upload.original_filename)
-        send_file(Discourse.store.path_for(upload), opts)
+
+        file_path = Discourse.store.path_for(upload)
+        return render_404 unless file_path
+
+        send_file(file_path, opts)
       else
         render_404
       end

spec/requests/uploads_controller_spec.rb

@@ -226,6 +226,14 @@ describe UploadsController do
       expect(response.status).to eq(404)
     end
 
+    it "returns 404 when the path is nil" do
+      upload = upload_file("logo.png")
+      upload.update_column(:url, "invalid-url")
+
+      get "/uploads/#{site}/#{upload.sha1}.#{upload.extension}"
+      expect(response.status).to eq(404)
+    end
+
     it 'uses send_file' do
       upload = upload_file("logo.png")
       get "/uploads/#{site}/#{upload.sha1}.#{upload.extension}"