MDL-33144 display filetype icon and mimetype based on extension

MDL-33144 display filetype icon and mimetype based on extension

- use finfo to determine mimetype of a file only when it is unknown from file extension
- display filetype icon and mimetype in filemanager based on file extension
- fixed a small issue with specifying extension instead of group in file_get_typegroup()
- allow only web-compartible image types when embedding into editor or using in database field
- created new groups web_video and web_audio to use in resource module