What if I want a descriptor to hold ASCII text?

There are some cases where you do need to hold 8-bit text: MIME types, or binary data, for example. To do this, you should explicitly use the 8 bit descriptors:

_LIT8(KImageGif,"image/gif");
TBufC8<15> mimeType(KImageGif);

further reading http://descriptors.blogspot.com/