AssimpContext.ImportFileFromStream formatHint?

[trommlbomml]

As I have asynchronous framework loading data transparent from server/local I always would like to handle data as byte-array. Unfortunatly I found no detailed information about AssimpContext.ImportFileFromStream about the formatHint parameter. Furthermore, aiImportFileFromMemory() from the native code allows so send an empty string to find out format by itself but the managed level does not permit it. Can you tell me what I have to pass to the this argument?