It seems reasonable to assume that, as a French loan word, dossier would mean the same thing in both languages – all the more so when, like bouquet or rendez-vous, its final consonant is unvoiced (dos-ee-ey), à la française. In fact, however, French and English usages differ in subtle but important ways.
In French, the word describes any bundle of documents with a label on the spine (dos means "back"), what we call a file or folder in English:
By extension, dossier is often used metonymically to refer to the issue addressed, or the work entailed, by each of the folders piled up on your desk:
Le ministre des finances est intervenu sur le dossier important de la réforme fiscale. // The finance minister addressed the important issue of tax reform.
La justice a été saisie du dossier. // The matter is before the courts.
Chaque assistant social gère environ 120 dossiers. // Each social worker handles around 120 cases.
Je suis restée tard au bureau pour traiter un dossier urgent. // I stayed at work late to deal with an urgent job.
In English, a dossier is a specific kind of file containing detailed information about a person or topic, generally gathered in the course of an investigation. We learned recently that "Russian spies put together compromising dossiers on both Clinton and Trump" (The Guardian, 11/01/2017). As this example suggests, a dossier is more likely than a prosaic file to contain information of a secret or unflattering nature. I can only guess that the loan-word is preferred when the writer wishes to convey that the contents are out of the ordinary – more bouquet than bunch, more romantic rendez-vous than dull meeting.
Here are a few more common French idioms with dossier and suggested translations:
Dossier médical = Medical record
Dossier de presse = Press kit
Dossier de candidature = Application
Dossier des présences = Attendance record
Admission sur dossier = Selective admission