Basing on this tutorial, here a very basic way to do that:
$('your_trigger_element_selector').on('click', function(){
var data = new FormData();
data.append('input_file_name', $('your_file_input_selector').prop('files')[0]);
// append other variables to data if you want: data.append('field_name_x', field_value_x);
$.ajax({
type: 'POST',
processData: false, // important
contentType: false, // important
data: data,
url: your_ajax_path,
dataType : 'json',
// in PHP you can call and process file in the same way as if it was submitted from a form:
// $_FILES['input_file_name']
success: function(jsonData){
...
}
...
});
});
Don't forget to add proper error handling