Your file will not leave your own computer and no data from it is directly sent to findthatpostcode.
To protect your privacy this script does not send the postcodes found in your file. Instead a code is created based on the postcode, and this code is sent to findthatpostcode.
This code will correspond to a number of different postcodes (around 30), and all postcodes that match this code are sent back to the browser to be matched.
This provides some level of privacy protection, but in some circumstances it could still be possible to make a reasonable guess as to the postcodes used in the file by looking at the codes requested.
Therefore it is recommended that you think carefully before using this tool with any personal or sensitive data.