core: make sure base_data is bytes or Pdata

This was working fine (although not python3 ready), but was not tested

bt5/erp5_dms/TestTemplateItem/portal_components/test.erp5.testDms.py

@@ -72,6 +72,7 @@ from Products.ERP5Type import Permissions
 from DateTime import DateTime
 from ZTUtils import make_query
 import PyPDF2
+from OFS.Image import Pdata
 
 QUIET = 0
 
@@ -2560,6 +2561,24 @@ return 1
       self.assertEqual(len(subject_result), 1)
       self.assertEqual(subject_result[0].getPath(), document.getPath())
 
+  def test_base_convertable_uses_pdata_for_base_data(self):
+    document = self.portal.document_module.newContent(
+      portal_type='Spreadsheet',
+      file=makeFileUpload('import_big_spreadsheet.ods'))
+    self.tic()
+    # for large documents base_data is stored as Pdata
+    self.assertIsInstance(document.base_data, Pdata)
+    # the accessor unpacks to bytes
+    self.assertIsInstance(document.getBaseData(), bytes)
+
+    # for small documents, it's bytes directly
+    document = self.portal.document_module.newContent(
+      portal_type='Text',
+      file=makeFileUpload('TEST-en-002.odt'))
+    self.tic()
+    self.assertIsInstance(document.base_data, bytes)
+    self.assertIsInstance(document.getBaseData(), bytes)
+
   def test_base_convertable_behaviour_with_successive_updates(self):
     """Check that update content's document (with setData and setFile)
     will refresh base_data and content_md5 as expected.

product/ERP5/bootstrap/erp5_core/MixinTemplateItem/portal_components/mixin.erp5.BaseConvertableFileMixin.py

@@ -80,7 +80,7 @@ class BaseConvertableFileMixin:
   security.declareProtected(Permissions.AccessContentsInformation,
                                                                  'getBaseData')
   def getBaseData(self, default=_MARKER):
-    """Serialise Pdata into string
+    """Serialise Pdata into bytes
     """
     self._checkConversionFormatPermission(None)
     if default is _MARKER:
@@ -90,7 +90,7 @@ class BaseConvertableFileMixin:
     if base_data is None:
       return None
     else:
-      return str(base_data)
+      return bytes(base_data)
 
   security.declareProtected(Permissions.ModifyPortalContent, '_setBaseData')
   def _setBaseData(self, data):