Code to Make a Button that Downloads File Stored on SD Card


I am a new user to the Arduino world. I'll start off by describing my Arduino board setup: I have a Mega 2560 with the Ethernet Shield attached and a 4GB micro SD card inserted. Attached above the Ethernet Shield, I have a Prototype Shield with an infrared sensor attached. Finally, I have an I2C 20x4 LCD display to show real time data from the sensor. Basically, this whole system is a fancy way of counting objects that have passed in front of the sensor.

Now on to my sketch: My sketch starts by displaying the number of objects that have passed by it. At the same time, the sketch also creates a log.txt file on the SD card that stores the number of objects to a new line in the log file every minute. Later in the sketch, I have it create a basic HTML webpage. On the webpage, it shows the number of objects with a refresh button that when the user clicks, that page updates and shows the new total number of objects the sensor has detected.

Here's my question: I have created a button called "Download Log File" in HTML in the sketch and I want the user to have the ability to click that button and download a copy of the log.txt file to their local computer. What code do I need to make the button do this?