Cached thumbnail displayed after the file is replaced in the filemanager

As discussed with Marina, URLs realthumbnail and realicon do not change when the original file is replaced/ovewritten in the filemanager. This makes the browser to use the locally cached version of the image instead of re-fetching the new thumbnail. To fix this:

• filemanager has to reload the files list from the server (Marina will do this part)
• thumbnail URL must contain an additional parameter to identify the original's revision

As the stored_file object is available when constructing these URLs, we decided to use the contenthash of the original file as the identifier. (alternatively we might use the timemodified stamp).