*.db files are database files, may be supported by Microsoft Access or it may be a thumbnail file. If it is showing 'type unknown' then there must be a 'file association' problem. Right click on that file, go to propertiesd check the option-
"Opens with: *****" See what program is associated with it. If there is no program, then click "Change" tab and associate it with its proper program. Check the box (tick mark) written as- "Always use the selected program to open this kind of file"