When importing from CSV (purchase invoices etc) the popup file browser defults to all files (.). It would be easier to select the correct file from the local PC if you added a (*.csv) filter to the file browser.
currently your popup code (revealed by browser F12) is this:
<input type=“file” name=“FileUpload1” id=“FileUpload1” class=“Light-Headers” style=… (etc)
I think all you need to add is another attribute: accept=".csv"