Hi Sven, We encountered a ZnMimePart Content-Disposition header problem with a server expecting a space following the "form-data;" string. The diff here... https://www.diffchecker.com/ETIfCUzx * on the left shows the working request from the default command-line tool * on the right shows the non-working request from Pharo. The specifications are really unclear about the requirement or not for a space, so it may be reasonable to consider the fault lies with the server not being permissive enough, but I notice these examples all have space after semi colons... * https://tools.ietf.org/html/rfc7578 * https://www.ietf.org/rfc/rfc2388.txt so I wonder if this scenario might be widespread (and endemic to Rails apps which this was) and maybe its pragmatic to include the extra spaces to cut off potential issues (it took quite a while to isolate with the live service being HTTPS only.) Steps to reproduce are here... https://github.com/exercism/pharo/issues/96#issuecomment-421224203 In the meantime, once isolated it was simple to work around, but it was almost a show stopper, so I'm interested in your thoughts. cheers -ben