If you are getting CHARSET as US-ASCII even if the header has a Content-Type: field, make sure the header also has a MIME-Version: field.
For example, the following header will correcty report charset as KOI8-R
MIME-Version: 1.0
Content-Type: text/plain; charset="koi8-r"
Without the MIME-Version it will be reported as US-ASII