On the main sreen of the email client, there is a button that says "Browse Files". Click on it, then select "Other Attachments".
Keep in mind that you can only open a limited number of file types. You can transfer them to your PC if you want to view the ones that aren't supported by the Instinct. .
If you have a card reader, you can look at the files that way and remove them. On the phone itself, just go to your email application > click the icon on the upper left-hand side > Browse Files. That's it. You can look for your files in your SD card.
Most email attachments will not open directly out of an email. The only current one the device is able to open are Word documents. If you do not have a card reader you can also connect your device to pc using the USB cable. Then on your computer open up My Computer, and there will be a new drive in "Devices with removable storage". The Instinct will automatically be read as a mass storage device and you can open, delete and edit the attachement in this way.