diff --git a/Signal/src/ViewControllers/MessagesViewController.m b/Signal/src/ViewControllers/MessagesViewController.m
index 39dfe7f67..dbcd6d42f 100644
--- a/Signal/src/ViewControllers/MessagesViewController.m
+++ b/Signal/src/ViewControllers/MessagesViewController.m
@@ -2182,7 +2182,7 @@ typedef enum : NSUInteger {
 
 - (void)showAttachmentDocumentPicker
 {
-    NSString *allItems = (__bridge NSString *)kUTTypeData;
+    NSString *allItems = (__bridge NSString *)kUTTypeItem;
     NSArray<NSString *> *documentTypes = @[ allItems ];
     // UIDocumentPickerModeImport copies to a temp file within our container.
     // It uses more memory than "open" but lets us avoid working with security scoped URLs.
@@ -2209,13 +2209,45 @@ typedef enum : NSUInteger {
     NSData *attachmentData = [NSData dataWithContentsOfURL:url];
 
     NSString *type;
-    NSError *error;
-    [url getResourceValue:&type forKey:NSURLTypeIdentifierKey error:&error];
-    if (error) {
-        DDLogError(@"%@ Determining type of picked document at url: %@ failed with error: %@", self.tag, url, error);
+    NSError *typeError;
+    [url getResourceValue:&type forKey:NSURLTypeIdentifierKey error:&typeError];
+    if (typeError) {
+        DDLogError(
+            @"%@ Determining type of picked document at url: %@ failed with error: %@", self.tag, url, typeError);
         OWSAssert(NO);
     }
 
+    NSNumber *isDirectory;
+    NSError *isDirectoryError;
+    [url getResourceValue:&isDirectory forKey:NSURLIsDirectoryKey error:&isDirectoryError];
+    if (isDirectoryError) {
+        DDLogError(@"%@ Determining if picked document at url: %@ was a directory failed with error: %@",
+            self.tag,
+            url,
+            isDirectoryError);
+        OWSAssert(NO);
+    } else if ([isDirectory boolValue]) {
+        DDLogInfo(@"%@ User picked directory at url: %@", self.tag, url);
+        UIAlertController *alertController = [UIAlertController
+            alertControllerWithTitle:
+                NSLocalizedString(@"ATTACHMENT_PICKER_DOCUMENTS_PICKED_DIRECTORY_FAILED_ALERT_TITLE",
+                    @"Alert title when picking a document fails because user picked a directory/bundle")
+                             message:
+                                 NSLocalizedString(@"ATTACHMENT_PICKER_DOCUMENTS_PICKED_DIRECTORY_FAILED_ALERT_BODY",
+                                     @"Alert body when picking a document fails because user picked a directory/bundle")
+                      preferredStyle:UIAlertControllerStyleAlert];
+
+        UIAlertAction *dismissAction = [UIAlertAction actionWithTitle:NSLocalizedString(@"DISMISS_BUTTON_TEXT", nil)
+                                                                style:UIAlertActionStyleCancel
+                                                              handler:nil];
+        [alertController addAction:dismissAction];
+
+        dispatch_async(dispatch_get_main_queue(), ^{
+            [self presentViewController:alertController animated:YES completion:nil];
+        });
+        return;
+    }
+
     if (!type) {
         DDLogDebug(@"%@ falling back to default filetype for picked document at url: %@", self.tag, url);
         OWSAssert(NO);
diff --git a/Signal/src/ViewControllers/SignalAttachment.swift b/Signal/src/ViewControllers/SignalAttachment.swift
index 76e89810c..e4f38f65d 100644
--- a/Signal/src/ViewControllers/SignalAttachment.swift
+++ b/Signal/src/ViewControllers/SignalAttachment.swift
@@ -12,7 +12,6 @@ enum SignalAttachmentError: Error {
     case couldNotParseImage
     case couldNotConvertToJpeg
     case invalidFileFormat
-    case unknownType
 }
 
 extension SignalAttachmentError: LocalizedError {
@@ -30,8 +29,6 @@ extension SignalAttachmentError: LocalizedError {
             return NSLocalizedString("ATTACHMENT_ERROR_COULD_NOT_CONVERT_TO_JPEG", comment: "Attachment error message for image attachments which could not be converted to JPEG")
         case .invalidFileFormat:
             return NSLocalizedString("ATTACHMENT_ERROR_INVALID_FILE_FORMAT", comment: "Attachment error message for attachments with an invalid file format")
-        case .unknownType:
-            return NSLocalizedString("ATTACHMENT_ERROR_UNKNOWN_TYPE", comment: "Attachment error message for attachments with an invalid file format")
         }
     }
 }
@@ -114,10 +111,6 @@ class SignalAttachment: NSObject {
         self.dataUTI = dataUTI
         self.filename = filename
         super.init()
-
-        if self.mimeType == nil {
-            error = .unknownType
-        }
     }
 
     // MARK: Methods
@@ -174,7 +167,7 @@ class SignalAttachment: NSObject {
 
     // Returns the MIME type for this attachment or nil if no MIME type
     // can be identified.
-    var mimeType: String? {
+    var mimeType: String {
         if dataUTI == SignalAttachment.kOversizeTextAttachmentUTI {
             return OWSMimeTypeOversizeTextMessage
         }
@@ -182,7 +175,7 @@ class SignalAttachment: NSObject {
             return OWSMimeTypeUnknownForTests
         }
         guard let mimeType = UTTypeCopyPreferredTagWithClass(dataUTI as CFString, kUTTagClassMIMEType) else {
-            return nil
+            return OWSMimeTypeApplicationOctetStream
         }
         return mimeType.takeRetainedValue() as String
     }
diff --git a/Signal/translations/en.lproj/Localizable.strings b/Signal/translations/en.lproj/Localizable.strings
index 5b985982d..a4358a607 100644
--- a/Signal/translations/en.lproj/Localizable.strings
+++ b/Signal/translations/en.lproj/Localizable.strings
@@ -106,6 +106,12 @@
 /* Alert title when picking a document fails for an unknown reason */
 "ATTACHMENT_PICKER_DOCUMENTS_FAILED_ALERT_TITLE" = "Failed to choose document.";
 
+/* Alert body when picking a document fails because user picked a directory/bundle */
+"ATTACHMENT_PICKER_DOCUMENTS_PICKED_DIRECTORY_FAILED_ALERT_BODY" = "Signal can't handle that file as is. Try zipping it before sending.";
+
+/* Alert title when picking a document fails because user picked a directory/bundle */
+"ATTACHMENT_PICKER_DOCUMENTS_PICKED_DIRECTORY_FAILED_ALERT_TITLE" = "Unsupported File";
+
 /* An explanation of the consequences of blocking another user. */
 "BLOCK_BEHAVIOR_EXPLANATION" = "Blocked users will not be able to call you or send you messages.";