Filefield file extension constraints

[mjjeon]

It would be good to set acceptable file extensions in FileFields. If you set it only to accept txt or csv file formats, when a user tries to load a file with other file formats then appyter can alert the user can't load that file.

[u8sand]

Related to #40
Assuming you express your filename constraints as a regex, for example:

FileField(
  ...,
  constraint=r'\.(txt|csv|tsv)$',
  ...,
)

This will prevent things server side, but with #40 we can translate this into a front-end validator as well.