Style Guide

File input styling

To achieve a cross-browser styling of the "Add files" button, the file input field is made transparent and positioned on top of the fileinput-button label - see the CSS definitions for the "fileinput-button" class in jquery.fileupload.css.

Why isn't it possible to programmatically trigger the file input selection?

Most browsers prevent submitting files when the input field didn't receive a direct click (or keyboard) event as a security precaution. Some browsers (e.g. Google Chrome) simply prevent the click event, while e.g. Internet Explorer doesn't submit any files that have been selected by a programmatically triggered file input field.
Firefox 4 (and later) is so far the only browser with full support for invoking "click"-Events on a completely hidden (display: none) file input field.

Bootstrap UI

The current version of the plugin builds on version 3 of Twitter's Bootstrap toolkit for its look&feel.

Please note: If you want to use Bootstrap version 4 (and possibly later), you need to set the following option:

$('#fileupload').fileupload({
  showElementClass: 'show'
});

See also: https://github.com/blueimp/jQuery-File-Upload/issues/3537