Add support for storing the file extension in the encrypted data.

[maxacan]

Right now, the program just appends .clypher to the name of any encrypted files, leaving the original file extension untouched.

When decrypting, the .clypher extension is simply stripped out, leaving the original file extension.

However, if the file extension happens to be missing, or is deleted by renaming the file, the program would have no idea what the original extension was.

This can be fixed by adding the file extension to the start of the encrypted data.

[maxacan]

This will also, theoretically, lift any limitations with the file handler only decrypting files based on their file extensions.

Once this is implemented, any file handler should move to detecting files based on the encrypted data, and not based on file extension.