Filtering search on fileType is returning results

https://developer.api.autodesk.com/data/v1/projects/<valid-project-id>/folders/<valid-folder-id>/search?filter[fileType]=<valid-file-type>

But

Filtering search on attributes.fileName-contains is not returning results even though there are matches

https://developer.api.autodesk.com/data/v1/projects/<valid-project-id>/folders/<valid-folder-id>/search?filter[attributes.fileName]-contains=<valid-search-value>

What have I missed?
Thx for your patience,
K.