How can I open a file with an extension of DMS on macOS Catalina?

Files with a .dms extension have been reported to be files downloaded by Safari where Safari did not recognize the file type and assigned .dms by default.

If you know the kind of file it actually is, you can try replacing the .dms extension with the appropriate extension (.doc, .xlsx, .txt, .jpeg etc) and then attempt to reopen the file.

Maybe? How to open DMS file extension on Mac - Apple Community

Depends if you are asking about a:

• Amiga Disk Masher Image
• Renamed Downloaded File
• Dream Maker Script

Be more specific in your question. This may help you: https://fileinfo.com/extension/dms

Contact your doctor's office and see if they can tell you what kind of file it is. If they use Electronic Medical Records (EMR), it could be a format proprietary to that system.