Is it an Alfresco bug? If it is not a bug, in what cases is this supposed to happen? I uploaded this file via CMIS (empty text file uploaded by CmisSync). All other files seem to have a cmis:contentStreamFileName.
Well - the cm:content property is not a mandatory property so you can actually have documents without any content. In that case the content stream properties of CMIS can be null. So that is not a bug in the way Alfresco presents the document. The question is what potentially went wrong during the upload - there could be an issue there.
<cite>The repository MUST return this property with a non-empty value if the document has a content stream and the property filter does not exclude it. If the document has no content stream, the repository MUST return "not set".</cite>
Here even though the object has a content stream (see its cmis:contentStreamId) it returns a null cmis:contentStreamFileName value.