Check if handle_upload returns a Transfer (#3838)

PosixTransfer will return the result of Pathname.chown (number of files
affected by operation) in directories with setgid bit set, which will
be considered a true value, but can not be used for render
"transfer/show".

apps/dashboard/app/controllers/files_controller.rb

@@ -156,7 +156,7 @@ def upload
     request.env[Rack::RACK_TEMPFILES].reject! { |f| f.path == params[:file].tempfile.path } unless posix_file?
 
     @transfer = @path.handle_upload(params[:file].tempfile)
-    if @transfer
+    if @transfer.kind_of?(Transfer)
       render 'transfers/show'
     else
       render json: {}