You can attach multiple files to a lead.
To upload a file with a lead, click on the lead from the milestone.
Select the option Files from the lead details page. You can drag and drop a file on the drop files area or click on drop file area to browse and select file. Once you have successfully uploaded the file, it will be reflected on the lead details page.
You can also have a look on the leads in the milestones and that is also a shortcut way to add the files. Just click on the file button on the lead from milestones.
The popup will open then you can add the files.