Britney Spears' Assistant -- Husband Files for Divorce ... After Ugly Dress Controversy

Britney Spears' infamously ugly bridesmaid dress at her assistant's wedding last year did NOT help keep the marriage intact -- because TMZ has learned, the assistant's husband has filed for divorce.

Britney famously appeared at her assistant Brett Miller's wedding in January 2011 -- sporting a notoriously hideous blue bridesmaid dress (above) -- and now, Brett's husband, reality TV producer Frank Lionetti, has filed a petition to end the marriage ... citing "irreconcilable differences."

According to the docs, Brett and Frank were married on January 28, 2011 -- and separated on February 14, 2012. They have no children.

And there's this -- Frank wants spousal support and attorney's fees from his estranged wife (reminder: she's an assistant).