Omission of was

Question

Court dismissed the plea filed by the victim.

Above is the original sentence read by me in news paper

Can I write the sentence as below,

Court dismissed the plea (which was) filed by the victim.

Answer 1

Yes. The original form

Court dismissed the plea filed by the victim.

is a "reduced relative clause", which you have expanded as

Court dismissed the plea which was filed by the victim.

This reduction is explained at

Thoughtco "reduced clause"
"Reduce to an Adjective Phrase"

Answer 2

"Court dismissed the plea which was filed by the victim."

Here,"which was filed by the victim" is a relative clause modifying its antecedent "the plea" in the main clause.

You can omit the part "which was" to form a reduced relative clause :

"Court dismissed the plea filed by the victim."

Here, the adjective phrase, "filed by the victim", serves as the postmodifier of the NP "the plea".

Therefore, both sentences are correct. The postmodifier of the NP (the plea) is a finte clause in one sentence and a phrase or a non-finite clause in another sentence.

In modern grammar, the adjective phrase "filed by the victim" is treated as the Past participle non-finite clause.