unable to upload image file when creating a message
This isn't fixing the root issue, but I tend to think the best practice here is to load media within the Wordpress Media Gallery and link to it in CiviMail. Content is best put in the CMS. That way, you can use the media for say a blog post and for CiviMail. It's more efficient.

I think some day being able for Civi to query the WP media gallery would be most desirable.