GMail - Adding Attachments
There are two ways to add attachments to an email message: The "Add Attachment Tool" and "DragNDrop".
Add Attachment Tool
Navigate via the "Add Attachment" tool.
Multiple attachments can be chosen by depressing the 'Shift' key and choosing items that are next to each other in a group or 'Command' key (Mac) or 'Control' key (Windows) and selecting multiple items that are not next to each other (shown below) you would like to attach.
A more direct option is to drag any desired attachments to the attachment area.