Do not disclose the file system full path

For example, when uploading a file called "foo/bar.txt" twice, right now it shows the full directory (e.g., "File /your/system/directory/foo/bar.txt already exists"), instead of only showing "File foo/bar.txt already exists".

flask_admin/contrib/fileadmin.py

@@ -596,7 +596,8 @@ class FileAdmin(BaseView, ActionsMixin):
                            secure_filename(form.upload.data.filename))
 
         if op.exists(filename):
-            flash(gettext('File "%(name)s" already exists.', name=filename),
+            secure_name = op.join(path, secure_filename(form.upload.data.filename))
+            flash(gettext('File "%(name)s" already exists.', name=secure_name),
                   'error')
         else:
             self.save_file(filename, form.upload.data)