#46 MIME Type check

* adds file mime type check on NewDocumentForm
* adds file size check on NewDocumentForm
* adds is_xy_valid methods as classmethods on AbstractDocument
* adds translations for error messages
* updates help text on NewDocumentForm file field

konova/models/document.py

@@ -5,6 +5,7 @@ Contact: michel.peltriaux@sgdnord.rlp.de
 Created on: 15.11.21
 
 """
+import mimetypes
 import os
 
 from django.db import models
@@ -62,6 +63,15 @@ class AbstractDocument(BaseResource):
     file = models.FileField()
     comment = models.TextField()
 
+    _accepted_mime_types = {
+        mimetypes.types_map[".pdf"],
+        mimetypes.types_map[".jpg"],
+        mimetypes.types_map[".jpeg"],
+        mimetypes.types_map[".png"],
+    }
+    # _maximum_file_size in MB
+    _maximum_file_size = 15
+
     class Meta:
         abstract = True
 
@@ -82,3 +92,12 @@ class AbstractDocument(BaseResource):
             pass
         super().delete(using=using, keep_parents=keep_parents)
 
+    @classmethod
+    def is_mime_type_valid(cls, _file: str):
+        mime_type = _file.content_type
+        return mime_type in cls._accepted_mime_types
+
+    @classmethod
+    def is_file_size_valid(cls, _file):
+        max_size = cls._maximum_file_size * pow(1000, 2)
+        return _file.size <= max_size