The trick in this sentence is to mentally remove the phrase set off by commas and see what subject and verb are left underneath.
Taking out "along with two uniforms" leaves the core sentence: "An important file... is missing from the police station." Phrases introduced by words like "along with," "as well as," or "together with" add extra information but never change the number of the main subject. Since "file" is singular, the verb has to stay singular too: "is missing," not "are missing," no matter how many uniforms are mentioned in between.
Separately, check the article before "file." The word starts with the consonant sound "f," said as "eff," which is a vowel sound at the start when spoken, so it actually takes "an," not "a." Native pronunciation, not spelling, decides the article here.
Combining both checks, the sentence needs "an" before "file" and the singular verb "is missing" left untouched by the inserted phrase.
So, the correct answer is An important file, along with two uniforms, is missing from the police station.
Reading these four sentences aloud, one after the other, brings out a different snag in three of them.
Each of the other three readings snags at a different point, leaving one version that sounds completely natural from beginning to end.
So, the correct answer is An important file, along with two uniforms, is missing from the police station.