How to change properties of all attachments type .gds into application/gds

[Eelco de Koning]

On my space, if I upload files with extension .gds, the files' properties are set to application/octet-stream. Clicking on a link to such an attachment triggers a download dialogue. Next you must click open to start the associated local application. This correct, but it is a two-step method and thus impractical. I think that application/octet-stream is the default value for files that are unknown to the wiki provider.

And .gds files have meaning to my audience. When I change the property to application/gds, on our systems when clicking the link immediately the right application starts. So, I am looking for two things:
1. How do I change, within my space, the properties of all attachments of type .gds, and no others, into application/gds?
2. How do I arrange that all new uploads of type .gds automagically get property application/gds?