decode content disposition header using utf8 for ios, which by default interprets the header as ASCII

gistfile1.m

- (NSString*)getFilenameFromContentDisposition:(NSString*)contentDisposition {
    // filename is optionally quoted.
    //
    // note that if the disposition indicates the asterisk
    // notation - filename*= - this indicates an explicit encoding;
    // in that case we will drop through to just using the responses
    // suggestedFilename (don't want to deal with the complexity,
    // hopefully the system has reasonably behaviour when the
    // encoding is actually unambiguous)
    NSString* suggestedFilename = nil;
    
    NSRegularExpression* regex = [NSRegularExpression
                                  regularExpressionWithPattern:
                                  @"^attachment;\\s*filename=\"([^\"]*)\"|(.*)"
                                  options:0
                                  error:nil];
    NSArray* matches = [regex matchesInString:contentDisposition
                                      options:0
                                        range:NSMakeRange(0,
                                                          [contentDisposition length])];
    if ([matches count] > 0) {
        NSTextCheckingResult *match = [matches objectAtIndex:0];
        if ([match numberOfRanges] >= 2) {
            // contentDisposition is ascii encoded; process it as utf8
            NSString* suggestedFilenameAscii = [contentDisposition
                                                substringWithRange:[match rangeAtIndex:1]];
            // apparently there's actually no way of accessing the underlying
            // bytes in an NSString*, so we ask for an array of utf32 characters
            // which should not result in any transcoding of the existing data
            const char* data = [suggestedFilenameAscii cStringUsingEncoding:NSUTF32LittleEndianStringEncoding];
            
            if (!data) {
                return nil;
            }
            
            NSMutableData* repackedData = [NSMutableData dataWithLength:[suggestedFilenameAscii length]];

            for (int i=0; i < [suggestedFilenameAscii length] * 4; i += 4) {
                if (data[i] > 0xFF) {
                    // not actually just octets in unicode like we thought! bail!
                    return nil;
                }
                ((char*)[repackedData mutableBytes])[i >> 2] = data[i];
            }
            
            suggestedFilename = [[[NSString alloc] initWithData:repackedData
                                                       encoding:NSUTF8StringEncoding] autorelease];
            
            // don't allow filenames with slashes in them
            NSRange slashRange = ([suggestedFilename rangeOfCharacterFromSet:[NSCharacterSet characterSetWithCharactersInString:@"/"]]);
            
            if (slashRange.location == NSNotFound) {
                return nil;
            }
            
            NSLog(@"forced utf8 interpetation of contentDisposition: %@",
                  suggestedFilename);
        }
    }
    
    return suggestedFilename;
}