Make sure field files.mimetype is correct

Make sure that field stored_file::mimetype (and corresponding DB field) is updated each time:
when file is uploaded, renamed or content is replaced.

If it is possible to determine the type from the file (binary files have this information inside), use mimetype from inside the file (function finfo_file?). If it is a text file, get mimetype from file extension.

It is important to display the correct file type in filemanager/filepicker, to determine whether we can display file inline or not, whether we allow or not this type in this particular file area.