MultipartFormData ContentHeaderEncoding UTF-8

[apps4everyone]

Currently it is not possible to set in MultipartFormData the ContentHeader Encoding information for the server side.
Some server fall back to ISO-8859-1 and not use UFT-8 .

Therefore explicit UTF-8 info should be optional possible to set.

The parameters "filename" and "filename*" differ only in that "filename*" uses the encoding defined in RFC5987, allowing the use of characters not present in the ISO-8859-1 character set (ISO-8859-1).

https://www.rfc-editor.org/rfc/rfc6266#ref-ISO-8859-1
https://greenbytes.de/tech/webdav/rfc5987.html#examples

[apps4everyone]

Sorry for disturbing @jshier, maybe it is possible to re review. It is more about server side to tell them that UTF-8 is used. This change in my PR is optional and will not break current usage. Thank you