Add validator to file upload in corpus creation.

app/main/forms.py

 from flask_wtf import FlaskForm
-from wtforms import MultipleFileField, SelectField, StringField, SubmitField
+from wtforms import MultipleFileField, StringField, SubmitField
 from wtforms.validators import DataRequired, Length
 
 
@@ -8,12 +8,12 @@ class CreateCorpusForm(FlaskForm):
         'Description',
         validators=[DataRequired(), Length(1, 64)]
     )
-    files = MultipleFileField('Files')
+    files = MultipleFileField('Files', validators=[DataRequired()])
+    submit = SubmitField('Create corpus')
     title = StringField(
         'Title',
         validators=[DataRequired(), Length(1, 32)]
     )
-    submit = SubmitField('Create corpus')
     '''
     def validate_files(form, field):
         for file in form.files.getlist[field.name]: