This doesn't feel heaps cleaner...

mediaItem.swift

class MediaItem : MTLModel, MTLJSONSerializing {
    
    var mediaFileName : String!
    var mediaMD5String : String!
    var mediaFileURL : String!
    var mediaFileSize : Int!
    
    class func documentsDirectory() -> String{
        return NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0] as String
    }
    
    class func JSONKeyPathsByPropertyKey() -> [NSObject : AnyObject]! {
        return ["mediaFileName" : "file_name", "mediaMD5String" : "md5_string", "mediaFileURL" : "media_file_url", "mediaFileSize" : "media_file_size"]
    }
    
    class func allMediaItems(filePath: String) -> [MediaItem]?{
        let fileString = MediaItem.documentsDirectory().stringByAppendingPathComponent("mediaItems.json")
        var error: NSError?
        let jsonData = NSData(contentsOfFile: fileString)
        
        if let scanData = jsonData{
            let result = NSJSONSerialization.JSONObjectWithData(scanData, options: NSJSONReadingOptions.AllowFragments, error: &error) as [AnyObject]?
            if let actualError = error{
                println("There was a massive error with this error: \(error?.localizedDescription)")
            }else if let totalArray = result{
                var serializationError: NSError?
                let serializeResults = MTLJSONAdapter.modelsOfClass(MediaItem.self, fromJSONArray: totalArray, error: &serializationError) as [MediaItem]?
                if let anError = serializationError {
                    println("There was a serialization error: \(anError.localizedDescription)")
                }else if let resultArray = serializeResults {
                    return resultArray
                }
            }
        }
        println("Nothing returned but an empty array")
        return []
    }
    
}