After deleting can you still see the file when you query it in the db?
where file_name = 'app300.css'
If you do find it, it's somehow still in your database. If you can't find it, my guess would be to look at your browser. Try opening your application in an other browser.
I had the same problem a while ago and, unfortunately, never got an answer (https://forums.oracle.com/thread/2557640).
I ended up uploading the CSS with a new name, e.g. app300_1.css and referencing the new name.
Had already tried in Chrome, Firefox and IE. Same result in each.
You are correct though, the query indicates that the row still exists in workspace files.
I am assuming that neither recent updates to the file or the delete (via Shared Components/Cascading Style Sheets) have been working. Although I don't know why.
I was unable to figure out why the problem was happening.
Tried restarting the server. No luck. I tried deleting the app300.css file via SQL Manager (vs. Shared Components/Cascading Style Sheets) and did not have the privileges to do so.
So in the end I took the approach of creating an app300screen.css and referencing that in my page templates.
If your file still recides in the database, you might want to try deleting it with a procedure from the wwv_flow_api package:
set serveroutput on
select workspace_id into l_security_group_id
(p_security_group_id => l_security_group_id);
( p_name => 'app300.css'
, p_location => 'WORKSPACE'
, p_mode => 'REMOVE'
, p_type => 'CSS'
You can run this using SQL developer/ Toad or whatever program you use, but I don't think you can run in the SQL workshop of Apex.
Run it as workspace owner.
First part sets the apex engine to point to the correct workspace id in Apex, change "APEX040000" to match your version of Apex.
Second part is the procedure that should remove your file from the database.
Make sure you commit your changes and try to upload your css file again.