Multipart request to create entity and content in single request fails with http 400

[NielsCW]

Describe the bug
Performing a multipart/form-data request to the endpoint to create entity and content in single request fails with 400 Bad Request

To Reproduce

• Perform a httpie request using http --print HhBb --form <url> foo=bar content@test.txt
• The server responds with a http 400 Bad Request

The application fails at this line because the Content-Type header has value multipart/form-data;boundary=8c...37;charset=UTF-8

Expected behavior
Succeeding to create entity with a 201 Created

[tgeens]

Note there are 2 parameters appended to the Content-Type, which are not covered by spring-content mockmvc-based tests:

• boundary=... is appended by (any) httpclient, mockmvc does not, because it operates at a higher abstraction level
• charset=UTF-8 is automatically appended by OrderedCharacterEncodingFilter (via HttpEncodingAutoConfiguration) in an auto-configuration scenario, but is not loaded via MockMvcBuilders.webAppContextSetup(context).build()