.DMS File Extension
File Type 1Disk Masher Image
What is a DMS file?
Compressed Amiga disk image created using The Disk Masher (DMS); originally used by actual Amiga computers to archive floppy disks; now often used to create disk images that can be opened by Amiga emulators, such as Amiga Forever.
File Type 2Dream Maker Script
.DMS File Association 2
Script (i.e. "connect.dms") run by Dream Seeker, a client for playing online games; created by Dream Maker, part of the BYOND (Build Your Own Net Dream) suite of programs.
File Type 3Renamed Downloaded File
.DMS File Association 3
A file with a .dms extension is a file that has been downloaded and renamed by the Apple Safari web browser or the Apple Mail email client. It contains the contents of a file that did not have a file extension or was erroneously renamed when downloaded. The .dms extension is a result of a bug in Safari and Mail.
You most likely will only encounter this type of DMS file if you download a file that does not have a file extension in macOS with Safari or Mail. In some cases, Safari or Mail may also replace a valid extension with .dms or may append the .dms extension onto a file extension. For example, sample.pdf may become sample.dms or sample.pdf.dms when downloaded with Safari or Mail.
To open a DMS file, rename the .dms extension with the appropriate extension for the file type. For example, if you know you downloaded an .XLSX file that was renamed with the .dms extension, click on the extension to highlight it and change it back to .xlsx. Then, you can open the file with a program that supports XLSX files.
The bug is specific to Macs and has been reported across multiple versions of Mac operating systems, with some issues dating back to the early 2000s. While the Safari and Mail are the typical perpetrators, the .dms renaming may also occur with other web browsers and programs.