Add checkboxes to ease the process of uploading files

[Emilien Klein]

As we use this plugin to upload files to various servers once a ticket is ready to be tested online, adding checkboxes would enable to keep track of the files that have already been uploaded, and the ones that are left to upload.

However, those checkboxes should be hidden by default, and a click on a link should make them appear, so that they don't get in the way when they are not needed.