Use temporary calendar files

app/src/main/java/eu/faircode/email/AdapterMessage.java

@@ -1364,10 +1364,7 @@ public class AdapterMessage extends RecyclerView.Adapter<AdapterMessage.ViewHold
                             response.setMethod(Method.REPLY);
                             response.addEvent(ev);
 
-                            File dir = new File(context.getFilesDir(), "temporary");
-                            if (!dir.exists())
-                                dir.mkdir();
-                            File ics = new File(dir, "meeting.ics");
+                            File ics = File.createTempFile(Long.toString(attachment.id), ".ics", context.getCacheDir());
                             response.write(ics);
 
                             return ics;

app/src/main/java/eu/faircode/email/FragmentCompose.java

@@ -2179,7 +2179,7 @@ public class FragmentCompose extends FragmentBase {
                         EntityAttachment attachment = new EntityAttachment();
                         attachment.message = draft.id;
                         attachment.sequence = 1;
-                        attachment.name = ics.getName();
+                        attachment.name = "meeting.ics";
                         attachment.type = "text/calendar";
                         attachment.disposition = Part.ATTACHMENT;
                         attachment.size = ics.length();