Because Gmail is a web-based system, you can't drag a file attachment from one message to another. As a workaround, you can do the following: Open the message or conversation that contains the file attachment. If the file is attached to a single message, click Forward (from the drop-menu at the top of the message). If it's attached to a message in a conversation, click Forward all on the right. Delete all the "forwarded" content from original messages, which appears at the bottom of your new message. Note that the file attachment remains with the forwarded message. Then compose your new message and send it. Alternatively, you can download the attachment and then upload it to another message.