Use only if you send a file.
First we try to use MIME_Type, if installed, to detect the content type, else we check if ext/mime_magic is loaded and properly configured.
Returns PEAR_Error if:
| MIME_Type failed to detect a proper content type (HTTP_DOWNLOAD_E_INVALID_CONTENT_TYPE) |
| ext/magic.mime is not installed, or not properly configured (HTTP_DOWNLOAD_E_NO_EXT_MMAGIC) |
| mime_content_type() couldn't guess content type or returned a content type considered to be bogus by setContentType() (HTTP_DOWNLOAD_E_INVALID_CONTENT_TYPE) |