[#4886] Added check for csv uploads to the MimeTypeValidator

There is a problem defining that a file is text/plain or text/csv.

We decided to make some extra checks in order to be closer to that
conclusion because a lot of csv files are not created properly
(according to the proper structure, delimiters etc.) and are treated as
text/plain from magic library.

Backport-of: #4940

Author: vaszig

src/openforms/formio/api/validators.py

@@ -96,6 +96,16 @@ def __call__(self, value: UploadedFile) -> None:
             raise serializers.ValidationError(
                 _("The provided file is not a {file_type}.").format(file_type=f".{ext}")
             )
+        # gh #4886
+        # We need to accept text/plain as a valid MIME type for CSV files.
+        # If the file does not strictly follow the conventions of CSV (e.g. non-standard delimiters),
+        # may not be considered as a valid CSV.
+        elif (
+            value.content_type == "text/csv"
+            and mime_type == "text/plain"
+            and ext == "csv"
+        ):
+            return
         elif mime_type == "image/heic" and value.content_type in (
             "image/heic",
             "image/heif",

src/openforms/formio/tests/files/test-csv-file.csv

@@ -0,0 +1 @@
+123

src/openforms/formio/tests/test_validators.py

@@ -188,6 +188,20 @@ def test_multiple_valid_mimetypes_in_zip_files_are_transformed(self):
 
             validator(sample)
 
+    def test_allowed_mime_types_for_csv_files(self):
+        valid_types = ("text/csv", "text/plain")
+        csv_file = TEST_FILES / "test-csv-file.csv"
+        validator = validators.MimeTypeValidator()
+
+        for valid_type in valid_types:
+            sample = SimpleUploadedFile(
+                "test-csv-file.csv",
+                csv_file.read_bytes(),
+                content_type=valid_type,
+            )
+
+            validator(sample)
+
     def test_validate_files_multiple_mime_types(self):
         """Assert that validation of files associated with multiple mime types works