Hi Namneesh,
I assume you are referring to the CSV importer?

If so, that error can only happen if one of two things is happening.

Either the file you are uploading is not a CSV file, or your server is misconfigured and lacks the PHP Extensions required to verify file mime-types (this is what helps ensure that the file you are uploading is not malicious).

If you have access to cPanel, then log in, type in Select PHP Version into the search bar and select that. Then ensure that you are both using a modern version of PHP (WordPress itself recommends PHP v8.3 or higher), and that the fileinfo extension is enabled. Outside of that, you’ll need to contact your host for support.