I need to extract the date from my filename "abc_20190401" and put it as a title in my dashboard?
You would extract the date from the file in a search and then set the token using the results of that search. Then reference your token in the title.
Setting a token using a search: https://answers.splunk.com/answers/442254/how-to-set-a-token-from-a-base-search-in-my-dashbo.html
Referencing a token in a title: https://answers.splunk.com/answers/639501/referencing-token-value-in-panel-title.html
Is the file name the source? Or is it a field? Is the file in some folder?
The file name is the source. The file is in a folder.