how to delete mail attahments from phone memory ?

Hi everyone. My problem is, whenever i read an email attachment by my phone, it is saved automatically. I need to check many emails everyday with attachments, and my "attachments" in the "stogare" of the phone is 2.2 GB. How can i delete the attachments, or how can i make a setting that the attachments will not be saved automatically, just like on the computer ? You know on the computer, when you download an attachment, it is saved in a "temporary" folder, and when you close the web browser or when you turn off your computer, they are deleted. But in iphone, it is not deleted. Any attachment which i read months ago, still on the phone memory.

Today i tried : "Mail" icon, "edit" on the up right, i marked "attachments" so it showed all the attachments. I deleted some, but i discovered that what is deleted is not the attachment file which is saved on the phone memory, but the email massage itself together with the attachment. It is deleted not from the phone but from the mail server, so when i check by computer, i also cannot find. So i had to "recover" all of them again from the "trash box" of the email server, so it is no use, my "attachments" is still 2.2 GB which is getting bigger day by day. Please everyone, show me a way, i have only few hundreds MB free storage on my 16 G phone, any "mail attahments" is 2.2 GB. Thanks in advance.