You shouldn't check "Negate validation" since regular behaviour of validator Validation based on regular expression is to validate that selected field's (or parsed text's) value matches entered regular expression. Look at the image attached to this transition. It shows a configuration that works works fine to force existence of at least a file attached to an issue.
You may also find useful knowing that you can also validate the type of files attached. Let's suppose we want to validate that our issue has the following files attached:
Exactly 1 XML file
To implement this validation we will use Boolean validator with math, date-time or text-string terms with the following configuration:
Boolean expression used is:
matches(%{00072}, "(.*application/java-archive.*){1,}") AND matches(%{00072}, "(.*text/plain.*){0,5}") AND matches(%{00072}, "(.*text/xml.*){1}") AND matches(%{00072}, "(.*image/.*){3,}")
Note that:
To design this configuration ,I have previously used Copy a parsed text to a field post-function in order to see the value that field "Attachments with details" takes when I attach different file types. Then it's easy to write a regular expressions for matching desired values.
You can implement that validation using "Boolean validator with math, date-time or text-string terms" with the following boolean expression:
%{00017} = "Critical" IMPLIES %{00071} ~ "rootcause.txt"
Note that:
You can also use the following expression in order to allow case insensitive comparison for the name of the attachments:
%{00017} = "Critical" IMPLIES toLowerCase(%{00071}) ~ "rootcause.txt"
You should write the name of the priority exactly as it is.
Validation based on regular expression
Boolean validator with math, date-time or text-string terms