The moment you select multiple files, file becomes a cell array.
Just run this in your command window and select a few files to see the data formats. Don't put ";" so you see how it looks.
[file, path] = uigetfile("Select something", 'MultiSelect', 'on')
You cannot make a fullfile of file, because it is no longer a string, but a cell array. Of course, Matlab cannot concatenate path (a string) with file (a cell array).
This means you will have to loop through your array and use something like
By the way, the answer you so nicely link in your question, contains the above information if you would read Jan's answer carefully.