To attach a file, simply open a composition or response window and drag your files onto the bottom area of that window.

Pictures can be either added in-line into the email (by dragging them into the email itself), or as an attachment (by dragging them into the bottom area)

— Alternatively, you can simply click the little paperclip icon in the bottom left and choose your file to attach.

— And, to choose the size of a photo you past inline, just click the “Insert Picture” button and choose what size you want.

Please note that you can now attach attach a file (either inline or as an attachment) when you are using the quick reply feature.

Also, you can copy and paste attachments from other emails, for that just simply use CTRL+C when selecting attachment and then CTRL+V in the compose window. For graphic files you may also use right click - paste file(s) as attachment.