Multipart originates from MIME, an Internet standard that extends the format of emails. Multipart messages are a container for parts of any content-type.
For example, a multipart message may have a part containing text and a second part containing an image. This is what allows you to attach files to emails.
In the context of HTTP, multipart is most often used
with the multipart/form-data
content-type. This is
the content-type you have to use when you want browsers
to be allowed to upload files through HTML forms.
Multipart is of course not required for uploading files, it is only required when you want to do so through HTML forms.
A multipart message is a list of parts. Parts may contain either a multipart message or a non-multipart content-type. This allows parts to be arranged in a tree structure, although this is a rare case as far as the Web is concerned.
In the normal case, when a form is submitted, the
browser will use the application/x-www-form-urlencoded
content-type. This type is just a list of keys and
values and is therefore not fit for uploading files.
That's where the multipart/form-data
content-type
comes in. When the form is configured to use this
content-type, the browser will use one part of the
message for each form field. This means that a file
input field will be sent in its own part, but the
same applies to all other kinds of fields.
A form with a text input, a file input and a select choice box will result in a multipart message with three parts, one for each field.
The browser does its best to determine the content-type of the files it sends this way, but you should not rely on it for determining the contents of the file. Proper investigation of the contents is recommended.