SPRING REST: The request was rejected because no multipart boundary was found

The problem isn't in your code - it's in your request. You're missing boundary in your multipart request. As it said in specification:

The Content-Type field for multipart entities requires one parameter, "boundary", which is used to specify the encapsulation boundary. The encapsulation boundary is defined as a line consisting entirely of two hyphen characters ("-", decimal code 45) followed by the boundary parameter value from the Content-Type header field.

This and this posts should also be helpful.

@sermolaev is right in his answer.

I want to share my experience related to this problem. I've encountered this problem in Postman, but I could not understand the root cause for it for a long time. My request template seemed to be correct cause Postman included boundary in it...

Eventually I've discovered that when you're specifying Content-Type=multipart/form header by yourself, it overrides the one added automatically by Postman. And this leads to the same error as yours. My solution was as simple as removing Content-Type header.