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- on folder :
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The properties dialog box shows what the folder contains (recursively), but it
counts each subfolder as an item. It would be better to make the difference
between the folders and files.

For example :
+----folder_1
|  +---folder_2
|  |---file_1
|  |---file_2
|
|---file_3
|---file_4

Properties on folder_1 should show :
- 1 folder (size in o, ko, mo, go - option to choose)
- 4 files (size  in o, ko, mo, go - option to choose)
and not 5 elements (size) (actual behaviour - not very useful).

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- on mixed items :
-----------------
The same new behaviou should be good.
For the moment, "properties on mixed items" isn't recursive (is it a bug ?),
it's quite anormal compared to the actual folder properties behaviour.