In Jira Server Gliffy diagrams are wrapped within a zip container.
- Download the gliffy attachment to your filesystem.
- Rename (on your filesystem) the file suffix to .zip, e.g. drawing.gliffy -> drawing.zip.
- Unzip the .zip file on your filesystem.
- In the versions folder find the highest numbered version in the number sequence and rename the suffix to .gliffy, e.g. 5.gon -> drawing.gliffy.
- Import that gliffy file into draw.io either via the menu File -> Import, or drag and drop the file into the draw.io editor.