With import / export definitions, optionally ask for a file to process at runtime

| Q                | A
| ---------------- | -----
| Bug report?      | no
| Feature request? | yes
| BC Break report? | no
| RFC?             | no
| Branch?          | master

Currently you can hardcode the file to use for import/export. It would be cool if you had a popup when you press Import which asks you which file to process, pre-selecting the hardcoded one (if defined).