It is unique timestamp which Helical Insight adds whenever you download anything from it. It uses this format like _. If the report name is empty then you get that type of file name.
You can give specific file name for dashboard, there are 2 ways,
Server end : Enable a setting which forces the end user to provide the name of the downloaded file. You can do so by changing the tag called false (set this to true).
Report end : You need to set reportName variable before exporting this variable and that will do the trick.
Helical Insight Creator