In French, the most natural way to say “attach a file” is joindre un fichier. A file attachment is usually called a pièce jointe or a fichier joint.

Common ways to say it:

  • Joindre un fichier = to attach a file
  • Ajouter une pièce jointe = to add an attachment
  • Vous trouverez ci-joint... = you will find attached...

Example:

  • Je joins le document à ce courriel. = I’m attaching the document to this email.

So if you want the safest everyday phrasing, use joindre un fichier.