Corrected bug that put the UTI instead of mimeType in retrieved assets

Summary:
This pull request corrects a bug  found in `RCTAssetsLibraryRequestHandler` which used the asset UTI as the mimeType.

The code for finding the mimeType is based on the implementation used in `RCTImageLoader` and `RCTImageUtils`.
Closes https://github.com/facebook/react-native/pull/7349

Differential Revision: D3252796

Pulled By: nicklockwood

fb-gh-sync-id: b9303a99333e4744dfe23045f4a2755307305772
fbshipit-source-id: b9303a99333e4744dfe23045f4a2755307305772

RCTAssetsLibraryRequestHandler.m

@@ -11,6 +11,7 @@
 
 #import <AssetsLibrary/AssetsLibrary.h>
 #import <libkern/OSAtomic.h>
+#import <MobileCoreServices/MobileCoreServices.h>
 
 #import "RCTBridge.h"
 #import "RCTUtils.h"
@@ -53,10 +54,13 @@ RCT_EXPORT_MODULE()
 
       ALAssetRepresentation *representation = [asset defaultRepresentation];
       NSInteger length = (NSInteger)representation.size;
+      
+        
+      CFStringRef MIMEType = UTTypeCopyPreferredTagWithClass((__bridge CFStringRef _Nonnull)(representation.UTI), kUTTagClassMIMEType);
 
       NSURLResponse *response =
       [[NSURLResponse alloc] initWithURL:request.URL
-                                MIMEType:representation.UTI
+                                MIMEType:(__bridge NSString *)(MIMEType)
                    expectedContentLength:length
                         textEncodingName:nil];