I would rewrite this sentence in one of the following ways to avoid the confusion and ambiquity that the current sentence has. First way: Enclosed are the following: application form, job description, memo, orientation information, and a letter. The colon indicates there is a lot more coming and it justifies the plural verb. Or second way: Enclosed is an application form, a job description, a memo, orientation information, and a letter. Putting a or an in front of each item makes it clear that you are referring to each in the singular. So you use ‘is.’