File extensions allowed for upload

Set allowed file extensions for uploaded user files in file_filter_fields.

Syntax:

allowed_files = ext
or
allowed_files = ext1|ext2|ext3

Example:

In your form:
…
<input type="file" name="attach">
…

Config file:
allowed_files = gif|bmp|png
file_filter_fields = attach (Picture)

Result:        

Only *.gif, *.bmp, *.png files would be allowed for user upload.