Hello,

Yes, you are right. I am sorry, I did not realize it.
In that case, it would be better to use an OnAfterValidate event of the BizForm control to set Text property of the error label according to your needs.

You can set the Text property to some value directly or you can create and use a new resource string:

<data name="basicform.emptyfile" xml:space="preserve">
<value>Please upload a file!</value>
</data>

Example of the OnAfterValidate event:

void viewBiz_OnAfterValidate()
{
     ((CMS.ExtendedControls.LocalizedLabel)viewBiz.BasicForm.FieldErrorLabels["FileField"]).Text = "Please upload a file!";
     // OR
     ((CMS.ExtendedControls.LocalizedLabel)viewBiz.BasicForm.FieldErrorLabels["FileField"]).Text = ResHelper.GetString("basicform.emptyfile");
}

For more information about BizForm handler events, I would like to point to the following article: Bizforms customization possibilities

Best regards,
Michal Legen