diff --git a/Signal/src/ViewControllers/SignalAttachment.swift b/Signal/src/ViewControllers/SignalAttachment.swift
index 182cb92c2..62ba4a0f2 100644
--- a/Signal/src/ViewControllers/SignalAttachment.swift
+++ b/Signal/src/ViewControllers/SignalAttachment.swift
@@ -174,11 +174,31 @@ class SignalAttachment: NSObject {
         if dataUTI == SignalAttachment.kUnknownTestAttachmentUTI {
             return OWSMimeTypeUnknownForTests
         }
-        let mimeType = UTTypeCopyPreferredTagWithClass(dataUTI as CFString, kUTTagClassMIMEType)
-        guard mimeType != nil else {
+        guard let mimeType = UTTypeCopyPreferredTagWithClass(dataUTI as CFString, kUTTagClassMIMEType) else {
             return nil
         }
-        return mimeType?.takeRetainedValue() as? String
+        return mimeType.takeRetainedValue() as String
+    }
+
+    // Use the filename if known. If not, e.g. if the attachment was copy/pasted, we'll generate a filename
+    // like: "signal-2017-04-24-095918.zip"
+    var filenameOrDefault: String {
+        if let filename = filename {
+            return filename
+        } else {
+            let kDefaultAttachmentName = "signal"
+
+            let dateFormatter = DateFormatter()
+            dateFormatter.dateFormat = "YYYY-MM-dd-HHmmss"
+            let dateString = dateFormatter.string(from: Date())
+
+            let withoutExtension = "\(kDefaultAttachmentName)-\(dateString)"
+            if let fileExtension = self.fileExtension {
+                return "\(withoutExtension).\(fileExtension)"
+            }
+
+            return withoutExtension
+        }
     }
 
     // Returns the file extension for this attachment or nil if no file extension
diff --git a/Signal/src/util/ThreadUtil.m b/Signal/src/util/ThreadUtil.m
index 2eca11f87..c2a5de5b4 100644
--- a/Signal/src/util/ThreadUtil.m
+++ b/Signal/src/util/ThreadUtil.m
@@ -64,7 +64,7 @@ NS_ASSUME_NONNULL_BEGIN
                                     expiresInSeconds:(configuration.isEnabled ? configuration.durationSeconds : 0)];
     [messageSender sendAttachmentData:attachment.data
         contentType:attachment.mimeType
-        filename:attachment.filename
+        filename:attachment.filenameOrDefault
         inMessage:message
         success:^{
             DDLogDebug(@"%@ Successfully sent message attachment.", self.tag);