Unfortunately, we have no built-in way to deliver files as you described. However, it may be possible to do it using our Send Stored Codes feature, if you can store the files on your own server. Basically, your product's list of Stored Codes would be a list of those files' URLs on your server, then we would issue one code (URL) from that list to each buyer.
Each code (URL) can be up to 255 characters long; if your URLs need to be longer than that, or if you'd prefer to paste a list of just the filenames rather than full URLs, we could get clever with using our Templated Email feature to handle the part of the URL that doesn't differ, so the Stored Codes would only consist of the differing filenames.
For security, we'd recommend using random filenames, as sequential filenames could be susceptible to unauthorized access -- e.g., a crooked buyer who receives 12345.jpg might try the URL for 12346.jpg to see if that works too, so random filenames would avoid that. You could use http://random.org/strings to generate randomized filenames.