Johann-S solution works great. (And it looks like he created the awesome plugin)
The Bootstrap 4.3 documentation example page uses this plugin to modify the filename: https://github.com/Johann-S/bs-custom-file-input
Use the Bootstrap custom file input classes. Add the plugin to your project, and add this code in your script file on the page.
$(document).ready(function () {
bsCustomFileInput.init()
})