diff --git a/SignalMessaging/attachments/SignalAttachment.swift b/SignalMessaging/attachments/SignalAttachment.swift
index 868eda051..d1a590cba 100644
--- a/SignalMessaging/attachments/SignalAttachment.swift
+++ b/SignalMessaging/attachments/SignalAttachment.swift
@@ -792,7 +792,7 @@ public class SignalAttachment: NSObject {
             // Most people won't hit this because we convert video when picked from the media picker
             // But the current API allows sending videos that some Signal clients will not
             // be able to view. (e.g. when picked from document picker)
-//            owsFail("building video with invalid output, migrate to async API using compressVideoAsMp4")
+            owsFail("building video with invalid output, migrate to async API using compressVideoAsMp4")
         }
 
         return newAttachment(dataSource: dataSource,
@@ -801,6 +801,17 @@ public class SignalAttachment: NSObject {
                              maxFileSize: kMaxFileSizeVideo)
     }
 
+    public class func copyToVideoTempDir(url fromUrl: URL) throws -> URL {
+        let baseDir = SignalAttachment.videoTempPath.appendingPathComponent(UUID().uuidString, isDirectory: true)
+        OWSFileSystem.ensureDirectoryExists(baseDir.path)
+        let toUrl = baseDir.appendingPathComponent(fromUrl.lastPathComponent)
+
+        Logger.debug("\(self.logTag) moving \(fromUrl) -> \(toUrl)")
+        try FileManager.default.copyItem(at: fromUrl, to: toUrl)
+
+        return toUrl
+    }
+
     private class var videoTempPath: URL {
         let videoDir = URL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent("video")
         OWSFileSystem.ensureDirectoryExists(videoDir.path)
diff --git a/SignalServiceKit/src/Util/MIMETypeUtil.m b/SignalServiceKit/src/Util/MIMETypeUtil.m
index e779d2dbb..ff9f7e0de 100644
--- a/SignalServiceKit/src/Util/MIMETypeUtil.m
+++ b/SignalServiceKit/src/Util/MIMETypeUtil.m
@@ -211,19 +211,20 @@ NSString *const kSyncMessageFileExtension = @"bin";
 }
 
 + (BOOL)isSupportedVideoFile:(NSString *)filePath {
-    return [[self supportedVideoExtensionTypesToMIMETypes] objectForKey:[filePath pathExtension]] != nil;
+    return [[self supportedVideoExtensionTypesToMIMETypes] objectForKey:filePath.pathExtension.lowercaseString] != nil;
 }
 
 + (BOOL)isSupportedAudioFile:(NSString *)filePath {
-    return [[self supportedAudioExtensionTypesToMIMETypes] objectForKey:[filePath pathExtension]] != nil;
+    return [[self supportedAudioExtensionTypesToMIMETypes] objectForKey:filePath.pathExtension.lowercaseString] != nil;
 }
 
 + (BOOL)isSupportedImageFile:(NSString *)filePath {
-    return [[self supportedImageExtensionTypesToMIMETypes] objectForKey:[filePath pathExtension]] != nil;
+    return [[self supportedImageExtensionTypesToMIMETypes] objectForKey:filePath.pathExtension.lowercaseString] != nil;
 }
 
 + (BOOL)isSupportedAnimatedFile:(NSString *)filePath {
-    return [[self supportedAnimatedExtensionTypesToMIMETypes] objectForKey:[filePath pathExtension]] != nil;
+    return
+        [[self supportedAnimatedExtensionTypesToMIMETypes] objectForKey:filePath.pathExtension.lowercaseString] != nil;
 }
 
 + (nullable NSString *)getSupportedExtensionFromVideoMIMEType:(NSString *)supportedMIMEType
diff --git a/SignalShareExtension/ShareViewController.swift b/SignalShareExtension/ShareViewController.swift
index ada9bb84d..47ea8c284 100644
--- a/SignalShareExtension/ShareViewController.swift
+++ b/SignalShareExtension/ShareViewController.swift
@@ -450,7 +450,30 @@ public class ShareViewController: UINavigationController, ShareViewDelegate, SAE
         // TODO accept other data types
         // TODO whitelist attachment types
         // TODO coerce when necessary and possible
-        return promise.then { (url: URL) -> Promise<SignalAttachment> in
+        return promise.then { (itemUrl: URL) -> Promise<SignalAttachment> in
+
+            let url: URL = try {
+                // iOS converts at least some video formats (e.g. com.apple.quicktime-movie) into mp4s as part of the
+                // NSItemProvider `loadItem` API.
+                // However, for some reason, AVFoundation operations such as generating a preview image and playing
+                // the url in the AVMoviePlayer fail on these converted formats until unless we first copy the media
+                // into our container. (These operations succeed when resending mp4s received and sent in Signal)
+                //
+                // I don't understand why this is, and I haven't found any relevant documentation in the NSItemProvider
+                // or AVFoundation docs.
+                //
+                // I *did* verify that the size and sah256 sum of the original url matches that of the copied url.
+                // Perhaps the AVFoundation API's require some extra file system permssion we don't have in the
+                // passed through URL.
+                if MIMETypeUtil.isSupportedVideoFile(itemUrl.path) {
+                    return try SignalAttachment.copyToVideoTempDir(url: itemUrl)
+                } else {
+                    return itemUrl
+                }
+            }()
+
+            Logger.debug("\(self.logTag) building DataSource with url: \(url)")
+
             guard let dataSource = DataSourcePath.dataSource(with: url) else {
                 throw ShareViewControllerError.assertionError(description: "Unable to read attachment data")
             }
@@ -467,7 +490,8 @@ public class ShareViewController: UINavigationController, ShareViewDelegate, SAE
             }
 
             guard !SignalAttachment.isInvalidVideo(dataSource: dataSource, dataUTI: specificUTIType) else {
-                // TODO show progress with exportSession
+                // This can happen, e.g. when sharing a quicktime-video from iCloud drive.
+
                 let (promise, exportSession) = SignalAttachment.compressVideoAsMp4(dataSource: dataSource, dataUTI: specificUTIType)
 
                 // Can we move this process to the end of the share flow rather than up front?
@@ -488,8 +512,6 @@ public class ShareViewController: UINavigationController, ShareViewDelegate, SAE
                 return promise
             }
 
-            // DO NOT COMMIT
-//            specificUTIType = "com.apple.quicktime-movie"
             let attachment = SignalAttachment.attachment(dataSource: dataSource, dataUTI: specificUTIType, imageQuality: .medium)
             return Promise(value: attachment)
         }