Respond to CR.

// FREEBIE

src/Util/MIMETypeUtil.m

@@ -16,17 +16,25 @@ NSString *const OWSMimeTypeImagePng = @"image/png";
 @implementation MIMETypeUtil
 
 + (NSDictionary *)supportedVideoMIMETypesToExtensionTypes {
-    return @{
+    static NSDictionary *result = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        result = @{
             @"video/3gpp" : @"3gp",
             @"video/3gpp2" : @"3g2",
             @"video/mp4" : @"mp4",
             @"video/quicktime" : @"mov",
             @"video/x-m4v" : @"m4v"
         };
+    });
+    return result;
 }
 
 + (NSDictionary *)supportedAudioMIMETypesToExtensionTypes {
-    return @{
+    static NSDictionary *result = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        result = @{
             @"audio/aac" : @"m4a",
             @"audio/x-m4p" : @"m4p",
             @"audio/x-m4b" : @"m4b",
@@ -46,10 +54,15 @@ NSString *const OWSMimeTypeImagePng = @"image/png";
             @"audio/3gpp2" : @"3g2",
             @"audio/3gpp" : @"3gp"
         };
+    });
+    return result;
 }
 
 + (NSDictionary *)supportedImageMIMETypesToExtensionTypes {
-    return @{
+    static NSDictionary *result = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        result = @{
             @"image/jpeg" : @"jpeg",
             @"image/pjpeg" : @"jpeg",
             OWSMimeTypeImagePng : @"png",
@@ -58,23 +71,38 @@ NSString *const OWSMimeTypeImagePng = @"image/png";
             @"image/bmp" : @"bmp",
             @"image/x-windows-bmp" : @"bmp"
         };
+    });
+    return result;
 }
 
 + (NSDictionary *)supportedAnimatedMIMETypesToExtensionTypes {
-    return @{
+    static NSDictionary *result = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        result = @{
             @"image/gif" : @"gif",
         };
+    });
+    return result;
 }
 
 + (NSDictionary *)supportedBinaryDataMIMETypesToExtensionTypes
 {
-    return @{
+    static NSDictionary *result = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        result = @{
             OWSMimeTypeApplicationOctetStream : @"dat",
         };
+    });
+    return result;
 }
 
 + (NSDictionary *)supportedVideoExtensionTypesToMIMETypes {
-    return @{
+    static NSDictionary *result = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        result = @{
             @"3gp" : @"video/3gpp",
             @"3gpp" : @"video/3gpp",
             @"3gp2" : @"video/3gpp2",
@@ -84,9 +112,15 @@ NSString *const OWSMimeTypeImagePng = @"image/png";
             @"mqv" : @"video/quicktime",
             @"m4v" : @"video/x-m4v"
         };
+    });
+    return result;
 }
+
 + (NSDictionary *)supportedAudioExtensionTypesToMIMETypes {
-    return @{
+    static NSDictionary *result = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        result = @{
             @"3gp" : @"audio/3gpp",
             @"3gpp" : @"@audio/3gpp",
             @"3g2" : @"audio/3gpp2",
@@ -107,10 +141,15 @@ NSString *const OWSMimeTypeImagePng = @"image/png";
             @"m4b" : @"audio/x-m4b",
             @"m4p" : @"audio/x-m4p"
         };
+    });
+    return result;
 }
 
 + (NSDictionary *)supportedImageExtensionTypesToMIMETypes {
-    return @{
+    static NSDictionary *result = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        result = @{
             @"png" : OWSMimeTypeImagePng,
             @"x-png" : OWSMimeTypeImagePng,
             @"jfif" : @"image/jpeg",
@@ -123,12 +162,19 @@ NSString *const OWSMimeTypeImagePng = @"image/png";
             @"tif" : @"image/tiff",
             @"tiff" : @"image/tiff"
         };
+    });
+    return result;
 }
 
 + (NSDictionary *)supportedAnimatedExtensionTypesToMIMETypes {
-    return @{
+    static NSDictionary *result = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        result = @{
             @"gif" : @"image/gif",
         };
+    });
+    return result;
 }
 
 + (BOOL)isSupportedVideoMIMEType:(NSString *)contentType {
@@ -328,22 +374,46 @@ NSString *const OWSMimeTypeImagePng = @"image/png";
 
 + (NSSet<NSString *> *)supportedVideoUTITypes
 {
-    return [self utiTypesForMIMETypes:[self supportedVideoMIMETypesToExtensionTypes].allKeys];
+    OWSAssert([NSThread isMainThread]);
+    static NSSet<NSString *> *result = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        result = [self utiTypesForMIMETypes:[self supportedVideoMIMETypesToExtensionTypes].allKeys];
+    });
+    return result;
 }
 
 + (NSSet<NSString *> *)supportedAudioUTITypes
 {
-    return [self utiTypesForMIMETypes:[self supportedAudioMIMETypesToExtensionTypes].allKeys];
+    OWSAssert([NSThread isMainThread]);
+    static NSSet<NSString *> *result = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        result = [self utiTypesForMIMETypes:[self supportedAudioMIMETypesToExtensionTypes].allKeys];
+    });
+    return result;
 }
 
 + (NSSet<NSString *> *)supportedImageUTITypes
 {
-    return [self utiTypesForMIMETypes:[self supportedImageMIMETypesToExtensionTypes].allKeys];
+    OWSAssert([NSThread isMainThread]);
+    static NSSet<NSString *> *result = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        result = [self utiTypesForMIMETypes:[self supportedImageMIMETypesToExtensionTypes].allKeys];
+    });
+    return result;
 }
 
 + (NSSet<NSString *> *)supportedAnimatedImageUTITypes
 {
-    return [self utiTypesForMIMETypes:[self supportedAnimatedMIMETypesToExtensionTypes].allKeys];
+    OWSAssert([NSThread isMainThread]);
+    static NSSet<NSString *> *result = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        result = [self utiTypesForMIMETypes:[self supportedAnimatedMIMETypesToExtensionTypes].allKeys];
+    });
+    return result;
 }
 
 @end