Mail allows you to view, play and forward attachments in a huge number of common formats. The most common are:
- HTML
- Word
- TXT
- Excel
- vCard (You can save attached vCards to Contacts.)
- Standard photo formats.
- WAV
- Unprotected AAC
- MP3
- H.264 Video
- MP4 Video
- 3GP Video
- QT MOV
Sometimes attachments will not open if the sender uses RTF format for email. You will have to ask them to resend in HTML or plain text. Media attachments burned with a codec not supported by QT may also not play. Since attachments remain on the server, you can forward them, even the ones you can't open on your iPhone. The only attachment you can make directly from your iPhone is an image from your camera or Photos.