I have a JSP tag file that requires a "size" attribute:

<%@ attribute name="size" required="true" type="Long" %>

Because the type accidentally wasn't a FQN (it should have been a java.lang.Long), this error was generated:

JBWEB004193: Unknown attribute type (size) for attribute Number

The message doesn't make any sense. "size" isn't the attribute type and the attribute is not "Number". A possible fix:

JBWEB004193: Unknown type (Number) was declared for attribute (size)

PS: It's really important to talk about the "declaration" here because the tag file isn't mentioned at all in the exception. The Jasper compiler only reports the enclosing JSP, which makes it look like the problem is with the usage not the tag file itself.