Only clear Firefox file input when necessary

lib/capybara/selenium/nodes/marionette_node.rb

@@ -26,7 +26,8 @@ class Capybara::Selenium::MarionetteNode < Capybara::Selenium::Node
   end
 
   def set_file(value) # rubocop:disable Naming/AccessorMethodName
-    native.clear # By default files are appended so we have to clear here
+    # By default files are appended so we have to clear here if its multiple and already set
+    native.clear if multiple? && driver.evaluate_script("arguments[0].files", self).any?
     return super if browser_version >= 62.0
 
     # Workaround lack of support for multiple upload by uploading one at a time