In the first case, you are catching a checked exception -- the compiler can actually check to see if the exception is actually thrown, and hence, complain if it isn't.
In the second case, the Exception class is a super class for many exceptions. Including the RuntimeException class, which is an unchecked exception. These are a class of exceptions which can be thrown at any time, by the JVM.... takes a breath.... Since, the compiler can't confirm that an unchecked exception won't be thrown, and since, the Exception class is a super class of unchecked exceptions, it is allowed.
Because the compiler can see that FileNotFoundException cannot be thrown from the method as its a checked exception. The second snippet catches the parent class Exception, which will catch both checked and unchecked exceptions. So there is a chance (as far as the compile is concerned) that a runtime (unchecked) exception could occur in the try block, even though its empty. Hence the second one is allowed.