I updated to 5.10. I thought that xib editing was supported in this release. When I try to edit a xib, I get "This File Format is not supported". Launch screens work fine.
The release notes say this:
"Full Xib and Xib Splash Screen support"
"We're pretty excited to finally ship full support for Xib files in this release. You can edit and interact with xib files just like you already could with storyboard files. This includes full support for designing xib splash screens."