Clean up file name some on save

Rusty Bird points out that we don't want the save action to create
files outside the parent or hidden files.

// FREEBIE

src/org/thoughtcrime/securesms/util/SaveAttachmentTask.java

@@ -181,6 +181,8 @@ public class SaveAttachmentTask extends ProgressDialogAsyncTask<SaveAttachmentTa
       fileName = base + "." + extension;
     }
 
+    fileName = new File(fileName).getName();
+
     int  i    = 0;
     File file = new File(outputDirectory, fileName);
 
@@ -189,6 +191,10 @@ public class SaveAttachmentTask extends ProgressDialogAsyncTask<SaveAttachmentTa
       file = new File(outputDirectory, fileParts[0] + "-" + (++i) + "." + fileParts[1]);
     }
 
+    if (file.isHidden()) {
+      throw new IOException("Specified name would not be visible");
+    }
+
     return file;
   }