E-mail file attachments and binary newsgroup postings are encoded before being sent over the internet. They are decoded by the e-mail program on receipt.
Most of the time this happens automatically, but there are different formats used, such as:
- MIME for Multipurpose Internet Mail Extensions
- Binhex for the Macintosh
- UUencode for Unix to Unix Encode
Of these MIME is used the most extensively. If an attachment was encoded with a format that your e-mail program is incapable of decoding, you will have a pile of garbage at the end.