Adjusted the error handling responsibility.

So that parameter definitions can define optional parameters.

core/src/main/java/hudson/model/FileParameterDefinition.java

@@ -96,7 +96,7 @@ public class FileParameterDefinition extends ParameterDefinition {
     public ParameterValue createValue(CLICommand command, String value) throws IOException, InterruptedException {
         // capture the file to the server
         FilePath src = new FilePath(command.checkChannel(),value);
-        File local = File.createTempFile("hudson","parameter");
+        File local = File.createTempFile("jenkins","parameter");
         src.copyTo(new FilePath(local));
 
         FileParameterValue p = new FileParameterValue(getName(), local, src.getName());

core/src/main/java/hudson/model/ParameterDefinition.java

@@ -165,6 +165,9 @@ public abstract class ParameterDefinition implements
      * <p>
      * If a {@link ParameterDefinition} can't really support this mode of creating a value,
      * you may just always return null.
+     *
+     * @throws IllegalStateException
+     *      If the parameter is deemed required but was missing in the submission.
      */
     public abstract ParameterValue createValue(StaplerRequest req);
 

core/src/main/java/hudson/model/ParametersDefinitionProperty.java

@@ -141,8 +141,6 @@ public class ParametersDefinitionProperty extends JobProperty<AbstractProject<?,
         	ParameterValue value = d.createValue(req);
         	if (value != null) {
         		values.add(value);
-        	} else if ((value == null) && !(d instanceof FileParameterDefinition)) {
-        	    throw new IllegalArgumentException("Parameter " + d.getName() + " was missing.");
         	}
         }