Non-compliant Content-Type header for multipart media type

[ijakus]

Affects: 5.x

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Two months ago similar issue has been submitted but has already been fixed and released so I'm opening a new one.

I also came across a server that is not robust enough and fails when charset is used as a parameter inside Content-Type header for multipart media type. As specified by RFC7578, only boundary is a required parameter and there are no optional parameters.

I'm submitting an issue for FormHttpMessageConverter. Same logic applied as a fix for #25885 can probably be used in this situation also.

spring-framework/spring-web/src/main/java/org/springframework/http/converter/FormHttpMessageConverter.java

Line 478 in a109b4c

parameters.put("charset", this.charset.name());

Edit:
I know there is a workaround where one can use setMultipartCharset() to prevent charset from showing in header. In my opinion it would be better if Content-Type header was aligned with specification and it worked in same fashion as MultipartWriterSupport now works.