It would be helpful if we could control the types of files being uploaded via data forms by having the match and match-error properties available for the file field type. This would allow us to make sure the file type being uploaded matches the purpose of the field using regular expressions.

An alternative would be to add a file-type property where we could simply specify the file extensions allowed (for those of us who are challenged by regular expressions). This could work similar to the default-schema for the url field type. For example:

[[form]]
fields:
  type:
    label: Music type
    type: select
    values:
      0: Classical
      1: Country
      2: Folk
      3: Indie
      4: Jazz
      5: Pop
      6: Rock
    default: 6
  bandimage:
    label: Image
    type: file
    file-type: jpg|png|gif|bmp
    match-error: Allowed file types are jpg, png, gif and bmp images only
  bandwebsite:
    label: Band website
    type: url
  current:
    label: Currently Recording
    type: select
    values:
      0: "Yes"
      1: "No"
    default: 0
[[/form]]