Import multiple .mat files into GUi

Hello I am writing code for gui which will load multiple .mat files, by using uigetfile then process it as following.

1. 'Load' button will open up dialog which allows user to choose which files will be processed.(Done)
2. It will display the loaded files in 'File List' listbox.(Done)
3. Choosing the range of loaded files with 'pop-up' menu.(Files are numbered as same format. e.g.) file1, file2, file3...)
4. ' Extract' will be choosing range that user chose, then do the process(already have the code for the process, but I don't know how to choose those specific files to be processed)

The bolded lines are my main question is.

Thanks!