SOAPUI property expansion in attachment content

I want to do the following with SOAPUI 5.2.1:

• Create a test case template that sends a multipart REST request to a service, including one or more attachments that are XML files
• Create multiple other test cases that will call the test case template
• In the attachment of the test case template, I want to define properties like ${#TestCase#prop1}, ${#Project#prop2}, etc. that I'd like to be replaced by SOAPUI property expansion

Some further details:

• The content type of the attachments is set to text/xml (I also tried application/xml)
• The file name of the attachment is provided by a property
• The type is automatically set to UNKNOWN
• Other property values that are used for parameter values in the same request are passed correctly from the test cases that call the template

Something similar was asked ~5 years ago here, but there are no answers.

Do I miss something or is this simply impossible?