Allow to chose which attachment to actually upload

With the work done in XWIKI-18810 the attachments are not anymore immediately uploaded but only when clicking on save button. However, it's not possible to decide which attachments will be in the end uploaded.
For example if a user upload image1 and then image2: even if he later decides that image1 should not be in the page, it will be uploaded.
We'd need a way to actually be able to remove attachments from being uploaded before saving.