objectForKey:NSFileType returns NULL

I am working on a method that identifies a kindle out of other mounted disks on the users mac. If the kindle can't be identified by name, my app will decide which mounted disk is a kindle by going through and finding a disk containing file types the kindle uses (ex. .prc, .azw, .mobi, .mbp, etc). Here is the code:

    NSLog(@"Scanning every file");
    NSMutableArray *mountedDisks = [[NSMutableArray alloc] init];
    mountedDisks = [workspace mountedRemovableMedia];

    NSMutableArray *subpaths = [[NSMutableArray alloc] init];

    int currentSubpath;
    int proprietaryFilesFound;

    while ([mountedDisks count] > currentDisk && [mountedDisks count] != 0)
    {
        subpaths = [manager subpathsAtPath:[mountedDisks objectAtIndex:currentDisk]];
        currentSubpath = 0;
        proprietaryFilesFound = 0;
        NSLog(@"Entered outer loop");

         while ([subpaths count] > currentSubpath && [subpaths count] != 0 && [[[manager attributesOfFileSystemForPath:[mountedDisks objectAtIndex:currentDisk] error:NULL] objectForKey:NSFileSystemSize] longLongValue] / 1073741824 <= 5 && proprietaryFilesFound < 7)
        {
            NSLog(@"Scanning %@, filetype is %@", [subpaths objectAtIndex:currentSubpath], [[manager attributesOfItemAtPath:[subpaths objectAtIndex:currentSubpath] error:NULL] objectForKey:NSFileType]);
            if ([[[manager attributesOfItemAtPath:[subpaths objectAtIndex:currentSubpath] error:NULL] objectForKey:NSFileType] isEqual: @"azw"] || [[[manager attributesOfItemAtPath:[subpaths objectAtIndex:currentSubpath] error:NULL] objectForKey:NSFileType] isEqual: @"mbp"] || [[[manager attributesOfItemAtPath:[subpaths objectAtIndex:currentSubpath] error:NULL] objectForKey:NSFileType] isEqual: @"prc"] || [[[manager attributesOfItemAtPath:[subpaths objectAtIndex:currentSubpath] error:NULL] objectForKey:NSFileType] isEqual: @"mobi"])
            {
                proprietaryFilesFound++;
                NSLog(@"Proprieta开发者_Python百科ry file found");
            }
            currentSubpath++;
        }
        currentDisk++;
    }

Unfortunately, when I run the following line of code, NULL is returned.

[[manager attributesOfItemAtPath:[subpaths objectAtIndex:currentSubpath] error:NULL] objectForKey:NSFileType]

I have worked with C++ for a long time, but I am fairly new to objective c and cocoa, so any help would be greatly appreciate, and I apologize ahead of time if this is a noob question.

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Instead of passing NULL to the error parameter, pass a pointer to an NSError and see what error you get back. For example:

NSError *error;
[[manager attributesOfItemAtPath:[subpaths objectAtIndex:currentSubpath] error:&error] objectForKey:NSFileType];
NSLog(@"%@", error);

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i suggest to check your "currentSubpath" variable, when manipulating path strings, it's easy to forgot a (holy) slash ...