Merge branch 'hotfix/#5962-mime-empty-string-on-no-part'

Close #5962
Close #6157
Hotfix for UnderscoreNamingStrategy for 2.3, will be fixed properly in 2.4
#6207 - removing unused `@group` annotation
Merge branch 'hotfix/#6207-underscore-naming-strategy-not-studlycase' into develop

Close #6207
Forward Port #6207