How can you read a files MIME-type in objective-c

Question

I am interested in detecting the MIME-type for a file in the documents directory of my iPhone application. A search through the docs did not provide any answers.

Answer 1

I'm not sure what are the practices on iPhone, but if you're allowed to, I'd make use of UNIX philosophy here: use program file, which is the standard way to detect filetype on an UNIX operating system. It includes a vast database of magic markers for filetype detection. Since file is probably not shipped on iPhone, you could include it in your app bundle. There might be a library implementing file's functionality.

Alternatively, you could trust the browser. Browsers send the MIME type they guessed somewhere in the HTTP headers. I know that I can easily grab the MIME type information in PHP. That of course depends if you're willing to trust the client.

Answer 2

Make sure are you import the coreservices

import <CoreServices/CoreServices.h>

in your file.

Answer 3

Based on the Lawrence Dol/slf answer above, I have solved the NSURL loading the entire file into memory issue by chopping the first few bytes into a head-stub and getting the MIMEType of that. I have not benchmarked it, but it's probably faster this way too.

+ (NSString*) mimeTypeForFileAtPath: (NSString *) path {
    // NSURL will read the entire file and may exceed available memory if the file is large enough. Therefore, we will write the first fiew bytes of the file to a head-stub for NSURL to get the MIMEType from.
    NSFileHandle *readFileHandle = [NSFileHandle fileHandleForReadingAtPath:path];
    NSData *fileHead = [readFileHandle readDataOfLength:100]; // we probably only need 2 bytes. we'll get the first 100 instead.

    NSString *tempPath = [NSHomeDirectory() stringByAppendingPathComponent: @"tmp/fileHead.tmp"];

    [[NSFileManager defaultManager] removeItemAtPath:tempPath error:nil]; // delete any existing version of fileHead.tmp
    if ([fileHead writeToFile:tempPath atomically:YES])
    {
        NSURL* fileUrl = [NSURL fileURLWithPath:path];
        NSURLRequest* fileUrlRequest = [[NSURLRequest alloc] initWithURL:fileUrl cachePolicy:NSURLCacheStorageNotAllowed timeoutInterval:.1];

        NSError* error = nil;
        NSURLResponse* response = nil;
        [NSURLConnection sendSynchronousRequest:fileUrlRequest returningResponse:&response error:&error];
        [[NSFileManager defaultManager] removeItemAtPath:tempPath error:nil];
        return [response MIMEType];
    }
    return nil;
}