mime

Package for working with MIME type definitions and for processing streams of MIME multipart media types.

Determining the MIME type for a file

The MimeTypeResolver class can be used to determine the MIME type of a file. It supports both using the extension of the file name and looking at magic bytes from the beginning of the file.

There is a builtin instance of MimeTypeResolver accessible through the top level function lookupMimeType. This builtin instance has the most common file name extensions and magic bytes registered.

import 'package:mime/mime.dart';

void main() {
  print(lookupMimeType('test.html'));
  // text/html

  print(lookupMimeType('test', headerBytes: [0xFF, 0xD8]));
  // image/jpeg

  print(lookupMimeType('test.html', headerBytes: [0xFF, 0xD8]));
  // image/jpeg
}

You can build you own resolver by creating an instance of MimeTypeResolver and adding file name extensions and magic bytes using addExtension and addMagicNumber.

Processing MIME multipart media types

The class MimeMultipartTransformer is used to process a Stream of bytes encoded using a MIME multipart media types encoding. The transformer provides a new Stream of MimeMultipart objects each of which have the headers and the content of each part. The content of a part is provided as a stream of bytes.

Below is an example showing how to process an HTTP request and print the length of the content of each part.

// HTTP request with content type multipart/form-data.
HttpRequest request = ...;
// Determine the boundary form the content type header
String boundary = request.headers.contentType.parameters['boundary'];

// Process the body just calculating the length of each part.
request
    .transform(new MimeMultipartTransformer(boundary))
    .map((part) => part.fold(0, (p, d) => p + d))
    .listen((length) => print('Part with length $length'));

Determining file extension by MIME type

The top level function extensionFromMime can be used to determine the file extension of a given MIME type.

print(extensionFromMime('text/html'));  // Prints "html".
print(extensionFromMime('image/jpeg'));  // Prints "jpg".
print(extensionFromMime('application/pdf'));  // Prints "pdf".