Fix a code comment in Upload->_file_mime_type()

Availability of dangerous functions is now tested using function_usable().
Fix typos in upgrade_300.rst
Add an upgrade note about change in the results of Directory Helper's directory_map()

see issue #1978
Merge pull request #2260 from vlakoff/develop-2

Add an upgrade note about change in the results of Directory Helper's directory_map()
Merge pull request #2258 from vlakoff/develop

Fix a code comment in Upload->_file_mime_type()
Fix form_upload() test
update