Just note that the ESP8266 can only send in a maximum chunk length (1426 off the top of my head), so I tend to chunk up any uploads into 1024 byte chunks, and in this case you'd want to read 1024 bytes at a time and send this to the host. There are examples of how to do this here, but once you've got yours sort of working, then post back here and we can help you with any problems.
If the "file" is quite short then one option is to convert this into a URI with a parameter and them use a simple HTTP client (again there are samples here and in the firmware distro), but note that you will need to escape some characters to make the URI -- again examples here and on the Internet.