I don't want to edit too much files to achieve this functionality to save customization on adding cart. For some ppl as a tip, you can and just "onchange" option f.e file upload. My webshop sells edible cake toppers from custom photos, at least 3-5 customers per day forgets to press "Save" button so order does not have uploaded picture and I need to bother customers to send photo as email so I needed to find a quick solution:
Open product.tpl file and find file upload form input line and add "onchange":
... <input onchange="javascript:this.form.submit();" type="file" name="file{$field.id_customization_field}" id="img{$customizationField}" class="form-control
customization_block_input ....
For me this solution works like charm. If you have time to write js-script, you can also make custom balloon popup to wait until photo is uploaded and visible on product.