api: get file extension from key

* closes zenodo/rdm-project#547

invenio_records_resources/records/api.py

@@ -10,6 +10,7 @@
 """Records API."""
 
 import mimetypes
+import os
 from contextlib import contextmanager
 
 from invenio_db import db
@@ -177,8 +178,16 @@ def dumps(self):
     @property
     def ext(self):
         """File extension."""
+        # The ``ext`` property is used to in search to aggregate file types, and we want e.g. both ``.jpeg`` and
+        # ``.jpg`` to be aggregated under ``.jpg``
         ext = mimetypes.guess_extension(self.object_model.mimetype)
-        return ext[1:] if ext else None
+        if ext is not None:
+            return ext[1:]
+
+        # Support non-standard file extensions that cannot be guessed
+        _, ext = os.path.splitext(self.key)
+        if ext and len(ext) <= 5:
+            return ext.lower()
 
     def __getattr__(self, name):
         """Override to get attributes from ObjectVersion and FileInstance."""