convert blob(image) to varchar in oracle

Question

I am trying to read blob (image) from an oracle db and display it in html. The image is larger than the buffer size so I have to split it first and then append all the sub-strings. My approach is as below (there will be a loop to go through the blob):

SELECT utl_raw.cast_to_varchar2(dbms_lob.substr(FILE_CONTENTS,2000,1)) as mystring from doc where file_name='test.png'

The problem is that the converted string looks scrambled

I did not specify the char_set for converting, could that be the reason? If so, how can I know which one to use?

Thanks.