allow to add custom extension to mime type mapping

[dbu]

ApacheMimetypeHelper has a hardcoded map of file extension to mime type map. It should be possible for the user to add custom mappings, overwrite mappings or remove mappings.

ApacheMimetypeHelper should have a constructor argument for additional mappings and merge those with its default mappings. Right now the only way to do this is extend ApacheMimetypeHelper and inject it with MultipartStreamBuilder::setMimetypeHelper

[Nyholm]

Today you could add your own extension by extending ApacheMimetypeHelper or to implement your own MimetypeHelper.

No matter if you extend or use a constructor argument, it could be tricky to inject them. Consider the Mailgun client: https://github.com/mailgun/mailgun-php/blob/master/src/Mailgun/Connection/RestClient.php#L85

[dbu]

ah right. still, i feel providing a list of additional mime types to the constructor of the mimetype helper would be simple and convenient. will adjust the description about my confusion about setting a custom helper.

this mailgun client should have a setter or constructor option for the mimetype helper to allow to inject one instead of it instantiating one on its own.