how to create test data using migration for file_column

The easy way to display this is to just store the binary data of the
file.

So you'd have a blob column called image...

Model.create(
  :name => 'john',
  :image => File.read("/Path/To/Image/File/john.jpg")
)

Check out http://www.mattberther.com/2007/10/19/uploading-files-to-a-database-using-rails/
for some image handling tips.