Computer side:
1. Log in to the QQ mailbox of the webpage and select Add Attachment in the main interface, as shown in the following figure:
2. Then select the file to send and open it, as shown in the following figure:
Mobile phone:
Open the QQ mailbox of your mobile phone, then select the+sign in the upper right corner, then select Write an email, and then select the attachment icon in the email, as shown in the following figure: