All English nouns beginning with the sound of a vowel use the article “an” and all English nouns beginning with the sound of a consonant use the article “a”. “Uniform” begins with a U which is a vowel, but it sounds like a “y” which is a consonant, so it uses the article “a”.

Это слово [ˈjuːnɪfɔːm] начинается со звука [j], который считается согласным.

It's not part of the word's spelling, but "uniform" begins with a "y" sound.