Error messages in RequestPartValidator prints typeof(DataType) instead of dataType.Id

[SandGrainOne]

Describe the bug

Example error:
"Error when comparing content to application metadata: The multipart section named vedleggDriftsplan has a Content-Type 'text/plain' which is invalid for element type 'Altinn.Platform.Storage.Interface.Models.DataType'."

The message contains the type name of DataType instead of the dataType.Id value.

To Reproduce

Steps to reproduce the behavior:

1. Perform a multipart request on POST {org}/{app}/instances where content type is invalid for at least one data element.
2. Results in Bad Request with the faulty error message.

Expected behavior

Error message includes the correct data type identifier.

[acn-sbuad]

nuget 4.9.1

[jeevananthank]

fix is verified and regression test on 4.9.1-alpha is complete. issue can be closed once the final nugets are pushed.

[SandGrainOne]

Final packages have been made and published.