Learn Turkish: Dosyanız burada.

The word dosya (file) ends in a vowel ('a'). Since the possessive suffix -ınız also starts with a vowel, we drop its first vowel to avoid having two vowels next to each other. Following 4-way vowel harmony rules for the 'a' in dosya, we just add -nız.

Is dosyanız plural ("your guys' file") or formal ("your file, sir")?

It can be either! The suffix -(ı)nız is used for the plural "you" (when speaking to multiple people), but it is also used as a polite, formal "you" when speaking to just one person. The context of the conversation will tell you which one it is.

Why does the word for "here" have the suffix -da on it?

The base word bura roughly means "this place". The -da at the end is the locative suffix, which means "in", "on", or "at". So burada literally means "at this place", which is the standard way to say "here" in Turkish.

Where is the word for "is" in this sentence?

Turkish doesn't use a separate word for "is" in simple "A is B" statements like this. Just saying Dosyanız burada (literally "Your file at this place") is a complete sentence meaning "Your file is here."