diff --git a/app/auth/forms.py b/app/auth/forms.py
index e8825aa21b56d41d08d3f3976624f6b2e8cdfca8..8d47e6b1a7b395c1864bbf60e93e4944ad774126 100644
--- a/app/auth/forms.py
+++ b/app/auth/forms.py
@@ -23,7 +23,7 @@ class RegistrationForm(FlaskForm):
     username = StringField('Username',
         validators=[
             InputRequired(),
-            Length(min=1, max=64),
+            Length(1, 64),
             Regexp(
                 USERNAME_REGEX,
                 message='Usernames must have only letters, numbers, dots or underscores'  # noqa
diff --git a/app/corpora/forms.py b/app/corpora/forms.py
index 8c2145952399678d1d353adb771b0e78de340ca8..8384f45bbbe6e2fa93d98a40768b5f59060adf5b 100644
--- a/app/corpora/forms.py
+++ b/app/corpora/forms.py
@@ -15,20 +15,20 @@ class AddCorpusFileForm(FlaskForm):
     Form to add a .vrt corpus file to the current corpus.
     '''
     # Required fields
-    author = StringField('Author', validators=[InputRequired(), Length(min=1, max=255)])
+    author = StringField('Author', validators=[InputRequired(), Length(1, 255)])
     publishing_year = IntegerField('Publishing year', validators=[InputRequired()])
-    title = StringField('Title', validators=[InputRequired(), Length(min=1, max=255)])
+    title = StringField('Title', validators=[InputRequired(), Length(1, 255)])
     vrt = FileField('File', validators=[FileRequired()])
     # Optional fields
-    address = StringField('Adress', validators=[Length(max=255)])
-    booktitle = StringField('Booktitle', validators=[Length(max=255)])
-    chapter = StringField('Chapter', validators=[Length(max=255)])
-    editor = StringField('Editor', validators=[Length(max=255)])
-    institution = StringField('Institution', validators=[Length(max=255)])
-    journal = StringField('Journal', validators=[Length(max=255)])
-    pages = StringField('Pages', validators=[Length(max=255)])
-    publisher = StringField('Publisher', validators=[Length(max=255)])
-    school = StringField('School', validators=[Length(max=255)])
+    address = StringField('Adress', validators=[Length(255)])
+    booktitle = StringField('Booktitle', validators=[Length(255)])
+    chapter = StringField('Chapter', validators=[Length(255)])
+    editor = StringField('Editor', validators=[Length(255)])
+    institution = StringField('Institution', validators=[Length(255)])
+    journal = StringField('Journal', validators=[Length(255)])
+    pages = StringField('Pages', validators=[Length(255)])
+    publisher = StringField('Publisher', validators=[Length(255)])
+    school = StringField('School', validators=[Length(255)])
     submit = SubmitField()
 
     def validate_vrt(self, field):
@@ -40,9 +40,9 @@ class EditCorpusFileForm(FlaskForm):
     Form to edit meta data of one corpus file.
     '''
     # Required fields
-    author = StringField('Author', validators=[DataRequired(), Length(1, 255)])
-    publishing_year = IntegerField('Publishing year', validators=[DataRequired()])
-    title = StringField('Title', validators=[DataRequired(), Length(1, 255)])
+    author = StringField('Author', validators=[InputRequired(), Length(1, 255)])
+    publishing_year = IntegerField('Publishing year', validators=[InputRequired()])
+    title = StringField('Title', validators=[InputRequired(), Length(1, 255)])
     # Optional fields
     address = StringField('Adress', validators=[Length(0, 255)])
     booktitle = StringField('Booktitle', validators=[Length(0, 255)])
@@ -60,8 +60,8 @@ class AddCorpusForm(FlaskForm):
     '''
     Form to add a a new corpus.
     '''
-    description = StringField('Description', validators=[DataRequired(), Length(1, 255)])
-    title = StringField('Title', validators=[DataRequired(), Length(1, 32)])
+    description = StringField('Description', validators=[InputRequired(), Length(1, 255)])
+    title = StringField('Title', validators=[InputRequired(), Length(1, 32)])
     submit = SubmitField()
 
 
@@ -69,14 +69,12 @@ class ImportCorpusForm(FlaskForm):
     '''
     Form to import a corpus.
     '''
-    description = StringField('Description', validators=[DataRequired(), Length(1, 255)])
-    archive = FileField('File', validators=[DataRequired()])
-    title = StringField('Title', validators=[DataRequired(), Length(1, 32)])
+    description = StringField('Description', validators=[InputRequired(), Length(1, 255)])
+    archive = FileField('File', validators=[FileRequired()])
+    title = StringField('Title', validators=[InputRequired(), Length(1, 32)])
     submit = SubmitField()
 
-    def __init__(self, *args, **kwargs):
-        super().__init__(*args, **kwargs)
-
     def validate_archive(self, field):
-        if field.data.mimetype != 'application/zip':
+        valid_mimetypes = ['application/zip', 'application/x-zip', 'application/x-zip-compressed']
+        if field.data.mimetype not in valid_mimetypes:
             raise ValidationError('ZIP files only!')
diff --git a/app/services/forms.py b/app/services/forms.py
index c5a9c7904bbcb4889c8641e70a35e36d37abc4d0..9d7bf45db8c3a978e0dea4996851132651f2705e 100644
--- a/app/services/forms.py
+++ b/app/services/forms.py
@@ -15,8 +15,8 @@ from . import SERVICES
 
 
 class AddJobForm(FlaskForm):
-    description = StringField('Description', validators=[InputRequired(), Length(min=1, max=255)])
-    title = StringField('Title', validators=[InputRequired(), Length(min=1, max=32)])
+    description = StringField('Description', validators=[InputRequired(), Length(1, 255)])
+    title = StringField('Title', validators=[InputRequired(), Length(1, 32)])
     version = SelectField('Version', validators=[DataRequired()])
     submit = SubmitField()