Unfortunately the ATTRIB command wasn't updated accordingly, so it cannot change this attribute.
There is a fantastic tool called FAR Manager which will allow you to change this and all other attributes:
FAR Manager (free and open source)
http://www.farmanager.com/
Within this program, you simply navigate to the file or directory for which you wish to change the attribute(s) for, then press CTRL-A -- "Encrypted" is one of the attributes you can change in the dialogue that will appear:
Additionally, if you select a number of files first (press the Insert key on your keyboard to select files individually, or use the + or - keys to select or de-select groups of files based on a pattern), you can then press CTRL-A to change the attributes for all of them at once.
HELPFUL TIP: When you're finished using this program, you can press F10 to exit.
For those who are interested, the full list of editable attributes are:
- Read only
- Archive
- Hidden
- System
- Compressed
- Encrypted
- Not indexed
- Sparse
- Temporary
- Offline
- Reparse point
- Virtual
...and other details that you can change include:
- Owner
- File modification date and time
- File creation date and time
- Last file access date and time