tab completion with user defined function

I am trying to write a function that takes a .txt file as an input, however the text files have long numerical names therefore I would like to be able to autocomplete them with tab. I have created a json file which I think is correct however when I try to use the function in the command window and tab in any files it just says there are no completions found. I was wondering if anyone can spot what is going wrong?

%json file

{
  "output_MT_data":
  {
     "inputs":
     [
        {"name":"filename",  "kind":"required", "type":"file=*.txt"}
     ]
  }
}

%function file

function output = output_MT_data(filename, counter_start, counter_end)

Table = readtable(filename);

if ~exist('counter_start', 'var')
    counter_start = 1;
end

if ~exist('counter_end', 'var')
    counter_end = height(Table);
end

output = Table(counter_start:counter_end, 1:4);

end

[Rik]

When I tried using functionSignatures, one of the suggestions was to restart Matlab to force a resfresh of the suggestions. Have you tried that?

[Ken Atwell]

The JSON file looks okay; where did you place this file in relation to the location of the function .m file?