Hi jberger,
Custom headers defined in the "encoding" attribute are not supported in "try it out" yet. You can follow this issue for status updates:
https://github.com/swagger-api/swagger-ui/issues/5356
As for Content-Length, RFC 7578 says:
>>
The multipart/form-data media type does not support any MIME header fields in parts other than Content-Type, Content-Disposition, and (in limited circumstances) Content-Transfer-Encoding. Other header fields MUST NOT be included and MUST be ignored.
<<
so this is not supported.
