for the document type column you must

1) ensure you have a column which is char(3) or char(4) and contains the
value htm or .htm
2) store your html content in an image data type column
3) use sp_fulltext_column to specify that the document type is specified in
the document type column you created  above in 1)

here is an example

sp_fulltext_column 'MyTable','ImageColumn',  'add', 1033,
'DocumentTypeColumn'

where MyTable is the table you are full text indexing, ImageColumn is a
column of the image datatype, and DocumentTypeColumn is the char(3) or
char(4) column which tells what the native type of the document you are
storing is.

Now, if you also might want to convert your docs to pure text. Using
FiltDump -b myhtmldoc.htm > myhtmldoc.txt is one way of doing it.

To convert your columns from html to text or from the text datatype to image
you should spit them out to the file system, and then convert them and push
them back.

Let me know if you need code samples to do this.