Yes, you can attach files to autoresponders, but only to the ones you have created. In the Notifications → Form User Options section of your form, check the option Send a confirmation message to the sender (autoresponder) (if not enabled yet) and hit Customize.
Click on Edit and check the option Attach a file to this autoresponder. This will reveal two options below: the possibility to attach the file from an URL or from your local device.
Make sure the file you want to attach by URL has the proper extension to be correctly uploaded and attached to your confirmation message/autoresponder (e.g. http://www.something-something.com/file.doc).
If you want to attach a local file, press Upload and select the file you want to attach to the autoresponder. Wait for the confirmation message to appear and click on Save to finish. Hit Close to exit lightbox. The file will be uploaded on our servers and can be accessed by the URL above the Upload button when returning to the customize lightbox.
Finally, form visitors will now receive the confirmation message with the file attached in their email, as seen in the example below.