Allows a file's content type to be provided explicitly, if known. Value is
expected to be a valid
media type.
For example, if you know your file is a JPEG image, then you should provide
the media type image/jpeg. If you don't know, or don't provide a media
type, a default type of application/octet-stream will be applied (which
indicates that the file should be regarded as pure binary data).
Options available when writing a file.