(fix) avoid exception on recent GNUstep when attached file has no filename (fixes #4702)

NEWS

@@ -11,6 +11,7 @@ Bug fixes
  - [core] allow super users to modify any event (#4216)
  - [core] correctly handle the full cert chain in S/MIME
  - [core] handle multidays events in freebusy data
+ - [core] avoid exception on recent GNUstep when attached file has no filename (#4702)
 
 4.0.7 (2019-02-27)
 ------------------

UI/MailPartViewers/UIxMailPartViewer.m

@@ -36,6 +36,7 @@
 #import <Mailer/NSData+Mail.h>
 #import <Mailer/NSDictionary+Mail.h>
 #import <Mailer/SOGoMailBodyPart.h>
+#import <Mailer/SOGoMailObject.h>
 
 #import "MailerUI/WOContext+UIxMailer.h"
 #import "UIxMailRenderingContext.h"
@@ -303,15 +304,14 @@
   NSMutableString *filename;
   NSString *extension;
 
-  filename = [NSMutableString stringWithString: [self filename]];
-  if ([filename length])
+  if ([[self filename] length])
     // We replace any slash by a dash since Apache won't allow encoded slashes by default.
     // See http://httpd.apache.org/docs/2.2/mod/core.html#allowencodedslashes
-    filename = [NSMutableString stringWithString: [filename stringByReplacingString: @"/" withString: @"-"]];
+    filename = [NSMutableString stringWithString: [[self filename] stringByReplacingString: @"/" withString: @"-"]];
   else
-    [filename appendFormat: @"%@-%@",
-	      [self labelForKey: @"Untitled"],
-	      [bodyPart nameInContainer]];
+    filename = [NSMutableString stringWithFormat: @"%@-%@",
+                               [self labelForKey: @"Untitled"],
+                                [bodyPart nameInContainer]];
 
   if (![[filename pathExtension] length])
     {