Under United States law: Using a trademark solely to refer to files compatible with a trademarked program would be allowed under the doctrine of nominative fair use.
The Ninth Circuit sets out the three-part test for nominative fair use in New Kids on the Block v. New America Pub, 971 F.2d 302 (9th Cir. 1992):
- "[T]he product or service in question must be one not readily identifiable without use of the trademark"
- "[O]nly so much of the mark or marks may be used as is reasonably necessary to identify the product or service"
- "[T]he user must do nothing that would, in conjunction with the mark, suggest sponsorship or endorsement by the trademark holder."
Simply referring to the name of a trademarked program to describe its file format (and not doing so in a way that suggests "sponsorship or endorsement by the trademark holder," such as slapping a "Certified Adobe Photoshop Compatible" sticker on the box or something like that) would meet these requirements.
The International Trademark Association elaborates a bit on the doctrine and gives some examples here.