https://www.openproject.org/docs/api/endpoints/attachments/
"/api/v3/work_packages/{id}/attachments

To add an attachment to a work package, a client needs to issue a request of type multipart/form-data with exactly two parts.

The first part must be called metadata. Its content type is expected to be application/json, the body must be a single JSON object, containing at least the fileName and optionally the attachments description.

The second part must be called file, its content type should match the mime type of the file. The body must be the raw content of the file. Note that a filename must be indicated in the Content-Disposition of this part, however it will be ignored. Instead the fileName inside the JSON of the metadata part will be used."

I have tried coding a solution to this, I've tried a mock in Postman. 

Possibly this is a language barrier, however, I nonetheless remain at a loss on how to upload a file and attach it to a workpackage via the API. 
In postman, the response I get is simply a "400 bad request":

https://***/api/v3/work_packages/23921/attachments
"form-data" checked. 

The below key value pairs: 

Key:                     Value: 

metadata            {"fileName":"test.png"}

file                       file.png

I would very much appreciate anyone's time in pointing me in the right direction. 

My eventual plan is to implement in Axios, but with a workable mock, I assume I can resolve this...