"decided upon by" is correct and can be used in written English.
It is used to refer to the decision-making process. For example: The regulations were decided upon by a committee of experts.
The editor of this series has a committee for selecting the pieces to be included, and ALL ART SHOULD BE JUDGED AND DECIDED UPON BY COMMITTEE.
They're decided upon by repute.
The precise level of punishment was decided upon by Duch.
"Huge swaths of policy and legislation will need to be reconsidered and decided upon by ministers, government and parliament.
The law says these awards will be decided upon by local district officials in negotiation with the local teachers unions.
For some promoted posts, the questions may be decided upon by a leeting panel, while some local authorities have standard question banks.
Northern Light uses categories displayed online in folders, categories with titles that are decided upon by people and that are continually being revised.
A shortlist of these bad films are decided upon by some 650 paid-up Razzie members in 19 countries – including a handful of celebrities Wilson won't name.
An urgent operation was decided upon by the three young surgeons on night service, and it was begun at 9 15 o'clock.
Yet a second consumption-tax hike, which must be decided upon by the end of the year, would test even his luck.
The spacing must be decided upon by the system designer.
