MIME Types

The e-mail script we made in part 3 is good enough for most web applications, but lacks a couple of important e-mail functions and features of modern software: MIME handling and attachments. MIME, Multipurpose Internet Mail Extensions, is a specification for enhancing the capabilities of standard e-mail. It allows the user to specify and encode a variety of media types for e-mail transmission. It also gives you more control over how the message should be interpreted.

Using MIME types, an e-mail can contain the following:

· Text messages in the US-ASCII character set.
· Character sets other than US-ASCII.
· Non-textual media including image, audio, and video.
· Binary files.
· Messages of unlimited length.


Content-type

The content-type header field is used to specify the type and subtype of data in the body of the message, and to specify the encoding of such data. Possible types include text, images, audio, video, multipart, applications, and many more.

If you don't specify a content-type, local e-mail programs will generally assume the following type:

Content-type: text/plain; charset=us-ascii

However, since no default value for a subtype is presumed, subtypes cannot be omitted as they have no significance alone. For example, they should be explicitly specified as text/html and image/jpg.


Content-transfer-encoding

Many e-mail ready content types are naturally represented as 8-bit character or binary data. However, such data cannot be transmitted over certain transport articles, such as SMTP, which restricts e-mail messages to 7-bit US-ASCII data with lines no longer than 1000 characters. This limitation, however, can be overcome by using the MIME Content-transfer-encoding header field. This header field is used to apply an encoding transformation to the message body - which will convert the native format to a protocal-friendly one.

The Content-transfer-encoding field's value is specified with a single type of encoding: 7bit, 8bit, binary, quoted-printable, or base64. The default value of a message body is 7bit and would be encoded as such:

Content-transfer-encoding: 7bit

The quoted-printable and base64 encoding types transform data into 7-bit format, making it safe to pass over restricted transports. The former type doesn't work reliably with some mail tranports, leaving base64 as the sole candidate for the most reliable encoding type for transferring non-textual data.


Now we're going to explore how to send attachments (files) using our e-mail script in Part 5 >>