Question : Why does EE not allow file uploads with the .accdb extension?

Why?

Access 2007 feels rejected.

Answer : Why does EE not allow file uploads with the .accdb extension?

perhaps, they have not included the .accdb as a secured file type.

to upload an .accdb format, (work around)
create a copy of the .accdb
change the .accdb extension to .txt
upload the file with the .txt extension