#385 Mimetype plugin in system module

[irmtfan]

currently there are many modules in xoops using their own defined Mimetype.
eg: system/images, news, publisher, xhelp, extgallery, xcgal, newbb, article, smartcontent, ...
now it is hardcoded in include\mimetypes.inc.php

Core team can define a mimetype plugin in system module and create a new table like this:
'mime_id'
'mime_name'
'mime_ext'
'mime_admin'
'mime_user'