Unable of attaching files to plain text mails in Yahoo! Mail

It looks like Photomail has been activated (don't remember how) on my
account, which makes that I can no longer attach a file to a mail in
Plain Text mode: in this mode, if I click on attach, I get a message
which says that I have to activate a mode which puts HTML in outgoing
mail (which I prefer to avoid); also, in this mode, I don't get the
standard Select File window of Internet explorer, but another window,
which seems to be yahoo!mail-specific (if i remember well, you have to
install a plugin to have this window). I saw another y!mail account
which can still attach files to mail in plain text; but on my account,
I didn't find an option in the preferences to get rid of these
Photomail things i'm not interested in. (Some time ago, I tried Yahoo!
mail beta for a few minutes and then I went back to the other version,
but I think it's a different problem which has nothing to do with
Photomail and attachments).