In "Activity_step3" I see the type being set as "TheMIMEType" which is placeholder text from the example.
In this case, you would want to use "text/plain" as the MIME type for that part.
As I do not know the structure expected by the endpoint, I cannot offer too much help on how your multipart needs to be structured, but a single text part may not be an appropriate request. The server may give you errors because you are sending fake data.
I cannot see step #4 in your screenshots, but commenting out any of the steps that build the multipart or create the request attachments page will cause you to either have an empty Multipart or not send a Multipart at all, so errors would definitely be expected in Pega or on the service side.
I also have similar requirement where needs to send case attachment along with meta data to the service. Using the REST connector and pyRequestAttachmentPage of Data-ServiceMessage class but it is giving me http 400 in response.