It's not Excel file, but MATLAB document has an Example of uigetfile and dropdown menu.
You can also open this example by the following command.
If you want to specify file extensions with .xlsx and read it as a table, the following will work.
[file, path] = uigetfile('*.xlsx');
t = readtable(fullfile(path, file));
I've attached a sample mlapp file (as a zip file) which sets drop down menu from input Excel file.