Creates a new MultipartFile from a chunked Stream of bytes. The length
of the file in bytes must be known in advance. If it's not, read the data
from the stream and use MultipartFile.fromBytes instead.
contentType currently defaults to application/octet-stream, but in the
future may be inferred from filename.
Source
MultipartFile(this.field, Stream<List<int>> stream, this.length,
{this.filename, MediaType contentType})
: this._stream = toByteStream(stream),
this.contentType = contentType != null ? contentType :
new MediaType("application", "octet-stream");