Import file, choose primary key column

[Roman2nc]

Is your feature request related to a problem? Please describe.

Currently, the first column is considered the primary key.

Users must be able to choose their primary key. The primary key can be optional; if there is none, it will not be possible to reimport another file into the same table.

[caoxing9]

Well, primary key will not effect re-import. There is no any primary key set when reimport, it's just key mapping. U can choose any parsed column to map original table field except some unsuport fieldtype like user,link(may support later).