I use [a function][1] in my theme that prevents upload of big size image files (thanks to @fischi), but I want to apply it **strictly** to .jpg/.jpeg files only, because it prevents to upload .pdf files too. How to adapt this function?

    add_filter('wp_handle_upload_prefilter', 'f711_image_size_prevent');
    function f711_image_size_prevent($file) {
    
        // get filesize of upload
        $size = $file['size'];
        $size = $size / 1024; // Calculate down to KB
    
        // get imagetype of upload
        $type = $file['type'];
        $is_image = strpos($type, 'image');
    
        // set sizelimit
        $limit = 700; // Your Filesize in KB
    
        // set imagelimit
        $imagelimit = 7;
    
        // set allowed imagetype
        $imagetype = 'image/jpeg';
    
        // query how many images the current user already uploaded
        global $current_user;
        $args = array(
            'orderby'         => 'post_date',
            'order'           => 'DESC',
            'numberposts'     => -1,
            'post_type'       => 'attachment',
            'author'          => $current_user->ID,
        );
        $attachmentsbyuser = get_posts( $args );
    
        if ( ( $size > $limit ) && ($is_image !== false) ) { // check if the image is small enough
            $file['error'] = 'Image files must be smaller than '.$limit.'KB';
        } elseif ( $type != $imagetype ) { // check if image type is allowed
            $file['error'] = 'Image must be ' . $imagetype . '.';
        } elseif ( count( $attachmentsbyuser ) >= $imagelimit ) { // check if the user has exceeded the image limit
            $file['error'] = 'Image limit of ' . $imagelimit . ' is exceeded for this user.';
        }
        return $file;
    
    }

**UPDATE**

This is the improved code (not tested yet), thanks to @nikhil-sheth:

    add_filter('wp_handle_upload_prefilter', 'f711_image_size_prevent');
    function f711_image_size_prevent($file) {

        // get imagetype of upload
        $type = $file['type'];
        $is_image = strpos($type, 'image');

        // set allowed imagetype
        $imagetype = 'image/jpeg';

        if ( $type == $imagetype ) {
    
        	// get filesize of upload
        	$size = $file['size'];
        	$size = $size / 1024; // Calculate down to KB
    
        	// set sizelimit
        	$limit = 700; // Your Filesize in KB
    
        	// set imagelimit
        	$imagelimit = 7;
    
        	// query how many images the current user already uploaded
        	global $current_user;
        	$args = array(
            		'orderby'         => 'post_date',
            		'order'           => 'DESC',
            		'numberposts'     => -1,
            		'post_type'       => 'attachment',
            		'author'          => $current_user->ID,
        	);
        	$attachmentsbyuser = get_posts( $args );
    
        	if ( ( $size > $limit ) && ($is_image !== false) ) { // check if the image is small enough
                $file['error'] = 'Image files must be smaller than '.$limit.'KB';
        	} elseif ( count( $attachmentsbyuser ) >= $imagelimit ) { // check if the user has exceeded the image limit
                $file['error'] = 'Image limit of ' . $imagelimit . ' is exceeded for this user.';
        	}
        	return $file;
        } else { return $file; }
    
    }

  [1]: http://wordpress.stackexchange.com/a/133527/25187