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