There is a known issue with WordPress 4.7.1 and 4.7.2 in regard to file uploads: https://core.trac.wordpress.org/ticket/39550
MemberMouse uses WordPress' file upload functionality. The import file is uploaded to the wp-content/uploads folder. The issue seems to be caused by files whose extension does not match the embedded mimetype.
As a workaround, you can put the file temporarily in the webroot of your server and just run the import "From URL" using the url http://yoursite.com/<filename>
Be sure to remove the file immediately afterward.
Another workaround mentioned in the WordPress Trac thread is a plugin that will remove the check mimetype filter:
*You are viewing this in our video library, for the full related article go to: Import Members with the Import Wizard
For all articles related to this topic, go to: Member Basics