Send email from action

Hi,
I am trying to create a simple Stateless form to send email to selected records.

the main form: 'Register' includes email field 'Email'
the Stateless form: 'Emailtoselection' includes mulltiline field 'RegID', and Email_Subject and Email_Message fields.

I basically followed this post where Cyril M explains and my code looks like this:

function to open Emailtoselection form and below that the sendmail function for the Send button:

void SendEmail_toselection(Register rec)
{
openUrl("#Form:Emailtoselection&RegID=" + (rec.ID.getall()), "Popup window", "height=400,width=600");
}

mailinglist=input.RegID.toList();
for each mID in mailinglist
{
    benefic = Register [ID == mID];
    sendmail
    [
        from:zoho.loginuserid
        to:benefic.Email
        subject:"input.Email_Subject"
        message:"input.Email_Message "
        content type:HTML
    ]
}

The first function works well and all ID,s are comma separated in the multiline field.
The second function gives an error, the fourth line benefic = Register [ID == mID] is invalid because ID is BIGINT and mID is a string. So how can I get the email IDs fetched with that mailinglist as reference?
Please help!

2. Also, if this is going to work, is there a way to attach a file to the email through the stateless form. Cannot add an upload field, since it is stateless, but is there another way, through an url maybe?