Why can't I save files to my iOS apps /Documents directory?

Why can't I save files to my iOS apps /Documents directory?

I am writing an app that can access the iOS root system, the user should
be able to save files to his document directory. I am using this code to
save the file to the document directory.
For Text:
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,
NSUserDomainMask,
YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
[self.filePath writeToFile:[NSString stringWithFormat:@"%@/%@",
documentsDirectory, [self.filePath lastPathComponent]]
atomically:YES
encoding:NSUTF8StringEncoding
error:nil];
For other files:
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,
NSUserDomainMask,
YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
[self.filePath writeToFile:[NSString stringWithFormat:@"%@/%@",
documentsDirectory,[self.filePath lastPathComponent]]
atomically:YES];
My problem is I can save .txt files, but not other files, If I save for
example .plist files with the save text methods, the contact is replaced
by the directory path of the file. When I save a picture, or any other
file it isn't readable. Is there a good method to save files without
destroying them?
--David