Check if the upload file name contains .gif If it does, refuse to upload it to the media gallery (by askjarvis.io)

functions.php

function wpse_check_file_type( $file ) {
    $filetype = wp_check_filetype( $file['name'] );
    $ext = $filetype['ext'];
    $type = $filetype['type'];
    $proper_filename = $file['name'];
    $wp_filetype = wp_check_filetype( $proper_filename, null );
    $ext = $wp_filetype['ext'];
    $type = $wp_filetype['type'];
    $proper_filename = preg_replace('/\.[^.]+$/', '', basename( $file['name'] ) );
    $filename = $proper_filename . '.' . $ext;
    if ( $type == 'image/gif' ) {
        $file['error'] = 'Sorry, GIF files are not allowed.';
        return $file;
    }
    return $file;
}
add_filter( 'wp_handle_upload_prefilter', 'wpse_check_file_type' );