HTTP协议上传boundary确定&下载content-disposition理解

<FORM ACTION="http://server.dom/cgi/handle"
ENCTYPE="multipart/form-data"
METHOD=POST>
What is your name? <INPUT TYPE=TEXT NAME=submitter>
What files are you sending? <INPUT TYPE=FILE NAME=pics>
</FORM>

The client might send back the following data:

Content-type: multipart/form-data, boundary=AaB03x

--AaB03x
content-disposition: form-data; name="field1"

Joe Blow
--AaB03x
content-disposition: form-data; name="pics"; filename="file1.txt"
Content-Type: text/plain

... contents of file1.txt ...
--AaB03x--

[RFC 1521] specifies a standard format for encapsulating multiple
pieces of data into a single Internet message. That document does not
address the issue of presentation styles; it provides a framework for
the interchange of message content, but leaves presentation issues
solely in the hands of mail user agent (MUA) implementors.

Two common ways of presenting multipart electronic messages are as a
main document with a list of separate attachments, and as a single
document with the various parts expanded (displayed) inline. The
display of an attachment is generally construed to require positive
action on the part of the recipient, while inline message components

are displayed automatically when the message is viewed.

disposition := "Content-Disposition" ":"
disposition-type
*(";" disposition-parm)

disposition-type := "inline"
/ "attachment"
/ extension-token
; values are not case-sensitive

disposition-parm := filename-parm / parameter

filename-parm := "filename" "=" value;