fixed up subclass check in to_mime

Author: tclose

fileformats/core/identification.py

@@ -128,11 +128,11 @@ def to_mime(
         namespace scheme instead of putting all non-standard types into the 'application'
         registry if not
     """
-    if not issubclass(datatype, fileformats.core.FileSet):
+    origin = ty.get_origin(datatype)
+    if origin is None and not issubclass(datatype, fileformats.core.DataType):
         raise TypeError(
             f"Cannot convert {datatype} to mime-type as it is not a file-set class"
         )
-    origin = ty.get_origin(datatype)
     if official and (origin or datatype.namespace == "field"):
         raise TypeError(
             f"Cannot convert {datatype} to official mime-type as it is not a proper "