Learn Turkish: Dosyaları kontrol ettiler mi?

In the witnessed past tense, personal endings (like -ler for "they") attach directly to the verb, not to the question particle. So you say ettiler mi? (did they check?), whereas in some other tenses you might be used to seeing the personal ending attach to the question word.

Why is there an -ı at the end of dosyaları?

The -ı is the accusative case ending, which marks a specific direct object. Because the sentence says "the files" (specific) rather than just "files" (general), you take the plural word dosyalar and add -ı.

Why are two words (kontrol ettiler) used to translate the single English word "check"?

In Turkish, many verbs are formed by combining a noun with a helper verb, most commonly etmek (to do / to make). Here, the noun kontrol is combined with etmek to create the action kontrol etmek (to check).

Why is the question particle mi and not mı?

The question particle changes to match the last vowel of the word right before it, following 4-way vowel harmony rules. Since the last vowel in ettiler is e, the particle must be mi.