I have several directories, each holding own or more files (with different file extensions!), something like this:

- Somedirectory
    - File1.txt
    - File2.doc
- AnotherDir
    - Banana.png
    - 1234.xls
- Somerandomname
    - Test1.txt
    - Test2.doc

And I watned this as a result:

- Somerandomname    (empty dir)
- Somedirectory.txt
- Somedirectory.doc
- AnotherDir     (empty dir)
- AnotherDir.png
- AnotherDir.xls
- Somerandomname     (empty dir)
- Somerandomname.txt
- Somerandomname.doc

 After some playing with Terminal/Shell (this will probably not work under Windows), this script will do that for all directories in the current directory:

for f in */* ;do fp=$(dirname "$f"); ext="${f##*.}" ; mv "$f" "$fp"."$ext" ;done

Hope this is useful to someone ...