I wanted to upload an excel file with one of my threads, but it is refused.
Do I have to zip it first and if so, why?

We currently do allow files with an .xlsx extension to e uploaded. What format was your spreadsheet in?

The extension is .xlsx, but I could not get it uploaded. Will try again and will let you know.

The error message I got is always (I tried again) The filetype you are attempting to upload is not allowed
I tried to post it as a "tutorial" it is just a display in excel of the rather complex hierachy of the DataGridView class.

Please email the file to me at dani@daniwebmail.com and I will try for myself. You shouldn't be getting that error message with an .xlsx file.

P.S. Attaching an image to this post just to make sure attachment uploading hasn't broken.