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.