Apache Reference: mod_expires, ExpiresByType
Generate Expires Header for a Particular Document MIME Type
Syntax: ExpiresByType MIME-type codeA
Example: ExpiresByType image/gif A2592000
Since: Apache 1.2
This directive defines the value of the Expires header
generated for documents of the specified MIME type. The
second argument specifies the number of seconds that will
be added to a base time to construct the expiration date. This
base time is either the time when the file was last modified or
the time when the client accessed the document. It is specified
by the code field, where ``M'' means that the file's last
modification time should be used as the base time, and ``A''
means the client's access time should be used. The difference in
effect is subtle. If ``M'' is used, all current copies of the
document in all caches will expire at the same time, which can
be good for a weekly notice that's always found at the same URL,
for example. If ``A'' is used, the date of expiration differs
for each client; this choice can be effective for image files
that don't change very often, for example, and particularly for
a set of related documents that all refer to the same images.