When first selecting a file for a deployment it is not immediately obvious that if you want a file to be a migration you must first enter a valid ‘File Name on Destination’. Once a file name is entered here the dropdown shows Migration as an option.
In the Otto v3.x Migrator file, a Migration was the default behaviour so it was a bit confusing for me that I couldn’t initially select migration as an operation for the file.
I suggest having migration appear in the drop down list but be visually clear that it is not selectable. When hovering over it a tooltip or similar could say something like “A destination file name must be provided for a migration”.