NullPointerException occures when Content type in MultiPart isn't sent

As written here:
http://www.rfc-editor.org/rfc/rfc2388.txt

Content type in MultiPart should be optional with default "text/plain" :

" As with all multipart MIME types, each part has an optional
"Content-Type", which defaults to text/plain. If the contents of a
file are returned via filling out a form, then the file input is
identified as the appropriate media type, if known, or
"application/octet-stream". If multiple files are to be returned as
the result of a single form entry, they should be represented as a
"multipart/mixed" part embedded within the "multipart/form-data".