BigFile: Factor out code to append data chunk to ._appendData()

So that data could be appended on server code via direct calls too.

NOTE previously ._read_data() accepted data=None argument and callers
were either providing it with current .data to append or None to forget
old content and just add new fresh one.

We could drop data=None from _read_data() signature, but leave it as is
for compatibility with outside code (e.g. zope's
OFS.Image.File.manage_upload() calls ._read_data(file) without any data
argument and in that case file content should be recreated, not
appended).

On the other hand we rework our code in .PUT() so for both "new content"
and "append range" in the end it always does "append" operation. For it
to work this way "new content" simply truncates the file before
proceeding to "append".
Reviewed-by: Romain Courteaud <romain@nexedi.com>

product/ERP5/Document/BigFile.py

@@ -344,7 +344,8 @@ class BigFile(File):
 
     content_range = REQUEST.get_header('Content-Range', None)
     if content_range is None:
-      btree = None
+      # truncate the file
+      self._baseSetData(None)
     else:
       current_size = int(self.getSize())
       query_range = re.compile('bytes \*/\*')
@@ -380,24 +381,29 @@ class BigFile(File):
           RESPONSE.setStatus(400)
           return RESPONSE
 
-        else:
-
-          btree = self._baseGetData()
-
       else:
         RESPONSE.setHeader('X-Explanation', 'Can not parse range')
         RESPONSE.setStatus(400) # Partial content
         return RESPONSE
 
-    data, size = self._read_data(file, data=btree)
-
-    content_type=self._get_content_type(file, data, self.__name__,
-                                        type or self.content_type)
-    self.update_data(data, content_type, size)
+    self._appendData(file, content_type=type)
 
     RESPONSE.setStatus(204)
     return RESPONSE
 
+
+  def _appendData(self, data_chunk, content_type=None):
+    """append data chunk to the end of the file
+
+       NOTE if content_type is specified, it will change content_type for the
+            whole file.
+    """
+    data, size = self._read_data(data_chunk, data=self._baseGetData())
+    content_type=self._get_content_type(data_chunk, data, self.__name__,
+                                        content_type or self.content_type)
+    self.update_data(data, content_type, size)
+
+
 # CMFFile also brings the IContentishInterface on CMF 2.2, remove it.
 removeIContentishInterface(BigFile)