I once had an app rejected for the same reason. After excluding downloaded files from being backed up it was approved. For setting that attribute I am using the following method:
// We do not want to backup this file to iCloud
+ (BOOL)addSkipBackupAttributeToItemAtURL:(NSURL *)URL {
const char* filePath = [[URL path] fileSystemRepresentation];
const char* attrName = "com.apple.MobileBackup";
if (&NSURLIsExcludedFromBackupKey == nil) {
// iOS 5.0.1 and lower
u_int8_t attrValue = 1;
int result = setxattr(filePath, attrName, &attrValue, sizeof(attrValue), 0, 0);
return result == 0;
} else {
// First try and remove the extended attribute if it is present
int result = getxattr(filePath, attrName, NULL, sizeof(u_int8_t), 0, 0);
if (result != -1) {
// The attribute exists, we need to remove it
int removeResult = removexattr(filePath, attrName, 0);
if (removeResult == 0) {
NSLog(@"Removed extended attribute on file %@", URL);
}
}
// Set the new key
return [URL setResourceValue:[NSNumber numberWithBool:YES] forKey:NSURLIsExcludedFromBackupKey error:nil];
}
}
You mentioned that using a cache was no option because you do not have control over if the files will remain in cache or not. You could solve that by using a custom cache. If you want to see a sample for that, then have a look at: https://github.com/evermeer/EVURLCache