Multiple upload document field

Appian OOTB doesn't support validating the extension on the form, instead you'll need to submit the form, perform the validation in the process model, and return to the form if it fails validation. See Eduardo's comment on this post: forum.appian.com/.../f-212684

Also check out this post of people having success with on-form file extension validation: forum.appian.com/.../e-153370

I've used on-form file validation (using the method we discussed in the thread Nick has linked, above) to great effect. I highly suggest breaking out the functionality into helper rules, particularly one which takes responsibility for retrieving the uploaded file properties (i.e. filename) from the temporary variable - just in case the behind-the-scenes code for this changes in the future.