diff --git a/uriloader/base/nsIDownload.idl b/uriloader/base/nsIDownload.idl
index b1b535171e9e..6373e0360e41 100644
--- a/uriloader/base/nsIDownload.idl
+++ b/uriloader/base/nsIDownload.idl
@@ -141,7 +141,29 @@ interface nsIDownload : nsITransfer {
 };
 
 %{C++
+/**
+ * A component with this contract ID will be created each time a download is
+ * started, and Init will be called on it and an observer will be set.
+ *
+ * Notifications of the download progress will happen via
+ * nsIWebProgressListener.
+ *
+ * If nsIObserver is implemented, the component may get a notification with
+ * topic "temp-file" and an nsILocalFile instance as subject, which indicates
+ * the location of a temporary file; i.e. a file in which the received data will
+ * be stored, but which is not equal to the target file.
+ *
+ * INTERFACES THAT NEED TO BE IMPLEMENTED:
+ *   nsISupports
+ *   nsITransfer
+ *   nsIDownload
+ *   nsIWebProgressListener
+ *
+ * INTERFACES THAT MAY BE IMPLEMENTED:
+ *   nsIObserver
+ */
 #define NS_DOWNLOAD_CONTRACTID "@mozilla.org/download;1"
+
 // {E3FA9D0A-1DD1-11B2-BDEF-8C720B597445}
 #define NS_DOWNLOAD_CID \
     { 0xe3fa9d0a, 0x1dd1, 0x11b2, { 0xbd, 0xef, 0x8c, 0x72, 0x0b, 0x59, 0x74, 0x45 } }
diff --git a/uriloader/exthandler/nsExternalHelperAppService.cpp b/uriloader/exthandler/nsExternalHelperAppService.cpp
index 0fe6f13a8255..a19683c56d6f 100644
--- a/uriloader/exthandler/nsExternalHelperAppService.cpp
+++ b/uriloader/exthandler/nsExternalHelperAppService.cpp
@@ -1845,6 +1845,11 @@ nsresult nsExternalAppHandler::InitializeDownload(nsIDownload* aDownload)
   
   rv = aDownload->SetObserver(this);
 
+  // Tell the listener about the location of the target file
+  nsCOMPtr<nsIObserver> obs(do_QueryInterface(aDownload));
+  if (obs)
+    obs->Observe(mTempFile, "temp-file", nsnull);
+
   return rv;
 }
 
@@ -2169,7 +2174,11 @@ NS_IMETHODIMP nsExternalAppHandler::Cancel()
   }
 
   // clean up after ourselves and delete the temp file...
-  if (mTempFile)
+  // but only if we got asked to open the file. when saving,
+  // we leave the file there - the partial file might be useful
+  nsMIMEInfoHandleAction action = nsIMIMEInfo::saveToDisk;
+  mMimeInfo->GetPreferredAction(&action);
+  if (mTempFile && action != nsIMIMEInfo::saveToDisk)
   {
     mTempFile->Remove(PR_TRUE);
     mTempFile = nsnull;