There's a good example (which includes file type validation) at:
https://github.com/mdn/learning-area/blob/master/html/forms/file-examples/file-example.html
It's basically a fleshed-out version Amos' idea of using a button.
I tried several of the suggestions above with no success (but maybe that's me).
I'm re-purposing it to do an Excel file conversion using
<form>
<div>
<label for="excel_converts">Choose a spreadsheet to convert.</label>
<input type="file" id="excel_converts" name="excel_converts" accept=".xlsx, .xls, .csv" >
</div>
<div class="preview">
<p>No spreadsheet currently selected for conversion</p>
</div>
<div>
<button>Submit</button>
</div>
</form>