Some customers had this issue in the past. So I decided to create this article to explain the behavior with OAG1.
Archive Link provides customization via OAC2 and OAC3, this way customer can maintain Document Classes at their will.
Sometimes the documents attached with that combination, does not display correctly, it might not open with desired application. Most of the cases I saw happen with Open Text Viewer, when SAP Archive Link does not carry the file extension.
This issue happens due to OAG1 settings. See note KB Article 1895984.