extract constant for image/png

// FREEBIE

src/Devices/OWSContactsOutputStream.m

@@ -2,6 +2,7 @@
 
 #import "OWSContactsOutputStream.h"
 #import "Contact.h"
+#import "MIMETypeUtil.h"
 #import "OWSSignalServiceProtos.pb.h"
 #import <ProtocolBuffers/CodedOutputStream.h>
 
@@ -20,7 +21,7 @@ NS_ASSUME_NONNULL_BEGIN
         OWSSignalServiceProtosContactDetailsAvatarBuilder *avatarBuilder =
             [OWSSignalServiceProtosContactDetailsAvatarBuilder new];
 
-        [avatarBuilder setContentType:@"image/png"];
+        [avatarBuilder setContentType:OWSMimeTypeImagePng];
         avatarPng = UIImagePNGRepresentation(contact.image);
         [avatarBuilder setLength:(uint32_t)avatarPng.length];
         [contactBuilder setAvatarBuilder:avatarBuilder];

src/Devices/OWSGroupsOutputStream.m

@@ -1,6 +1,7 @@
 //  Copyright © 2016 Open Whisper Systems. All rights reserved.
 
 #import "OWSGroupsOutputStream.h"
+#import "MIMETypeUtil.h"
 #import "OWSSignalServiceProtos.pb.h"
 #import "TSGroupModel.h"
 #import <ProtocolBuffers/CodedOutputStream.h>
@@ -21,7 +22,7 @@ NS_ASSUME_NONNULL_BEGIN
         OWSSignalServiceProtosGroupDetailsAvatarBuilder *avatarBuilder =
             [OWSSignalServiceProtosGroupDetailsAvatarBuilder new];
 
-        [avatarBuilder setContentType:@"image/png"];
+        [avatarBuilder setContentType:OWSMimeTypeImagePng];
         avatarPng = UIImagePNGRepresentation(group.groupImage);
         [avatarBuilder setLength:(uint32_t)avatarPng.length];
         [groupBuilder setAvatarBuilder:avatarBuilder];

src/Util/MIMETypeUtil.h

@@ -1,6 +1,7 @@
 //  Copyright © 2016 Open Whisper Systems. All rights reserved.
 
 extern NSString *const OWSMimeTypeApplicationOctetStream;
+extern NSString *const OWSMimeTypeImagePng;
 
 @interface MIMETypeUtil : NSObject
 

src/Util/MIMETypeUtil.m

@@ -4,6 +4,7 @@
 #endif
 
 NSString *const OWSMimeTypeApplicationOctetStream = @"application/octet-stream";
+NSString *const OWSMimeTypeImagePng = @"image/png";
 
 @implementation MIMETypeUtil
 
@@ -43,7 +44,7 @@ NSString *const OWSMimeTypeApplicationOctetStream = @"application/octet-stream";
     return @{
         @"image/jpeg" : @"jpeg",
         @"image/pjpeg" : @"jpeg",
-        @"image/png" : @"png",
+        OWSMimeTypeImagePng : @"png",
         @"image/tiff" : @"tif",
         @"image/x-tiff" : @"tif",
         @"image/bmp" : @"bmp",
@@ -102,8 +103,8 @@ NSString *const OWSMimeTypeApplicationOctetStream = @"application/octet-stream";
 
 + (NSDictionary *)supportedImageExtensionTypesToMIMETypes {
     return @{
-        @"png" : @"image/png",
-        @"x-png" : @"image/png",
+        @"png" : OWSMimeTypeImagePng,
+        @"x-png" : OWSMimeTypeImagePng,
         @"jfif" : @"image/jpeg",
         @"jfif" : @"image/pjpeg",
         @"jfif-tbnl" : @"image/jpeg",

src/Util/UIImage+contentTypes.m

@@ -1,11 +1,7 @@
-//
-//  UIImage+contentTypes.m
-//  Signal
-//
 //  Created by Frederic Jacobs on 21/12/14.
 //  Copyright (c) 2014 Open Whisper Systems. All rights reserved.
-//
 
+#import "MIMETypeUtil.h"
 #import "UIImage+contentTypes.h"
 
 @implementation UIImage (contentTypes)
@@ -18,7 +14,7 @@
         case 0xFF:
             return @"image/jpeg";
         case 0x89:
-            return @"image/png";
+            return OWSMimeTypeImagePng;
         case 0x47:
             return @"image/gif";
         case 0x49: