Create a Simulink Block that loads data from a file

I'd like to have a simulink block that allows me to select an input file and then modify parameters based on the content of the input file.

I've tried looking at Masks but am falling at the first hurdle with respect to not being able to add a button to the dialog.

There are plenty of blocks in the library that include buttons so these must be built with a different method than a mask. What is this method?