Hi @naokiajima1989
Hope you are doing fine!
I performed the same steps on my lab site but was not able to reproduce the issue. When uploading files, both single an multiple options worked fine.
The issue seems to be caused by a conflict between Forminator and other element in the site. Could you proceed a plugin/theme conflict test to identify which element could be causing this problem? Make sure you have recent backups and perform this operation on your dev/staging site if possible. To learn more about a plugin conflict test, please see this guide below:
https://wpmudev.com/docs/getting-started/getting-support/#conflict-test
Basically, you’ll need to deactivate all plugins, except Forminator and switch to a basic theme. Then test if the issue persists. If it doesn’t then start activating the plugins one by one until you identify the one causing the issue.
You can also check out this handy flowchart of the troubleshooting process detailed above.
https://wpmudev.com/wp-content/uploads/2015/09/Support-Process-Support-Process.gif
Please, let know the results so we can provide additional assistance based on the findings.
Kind regards
Luis