Validating file uploads in Flask Marshmallow

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This is not an issue, this is more of a question, I am sorry if I was not supposed to post it here but I couldn't find anything related to file uploading in the flask-marshmallow (or marshmallow) docs.

- Is there a way to validate file uploads in Flask Marshmallow?

Like uploading images and validate the mime type (if it's png, jpg, gif).

I saw people trying to use validators with no success:

```python
class UploadSchema(Schema):
image = fields.Raw(type="file", validate=FileExtensionValidation())
```

```python
class FileExtensionValidation(Validator)
def __call__(self, value, **kwargs):
....
```

But that lead to nowhere as I am not sure how to read the file submitted to check its name (cant do it from `value` in the example above).

Thanks in advance.

Start by reviewing the documented behavior of fields.Raw and the validator pattern shown in the issue, then inspect the Flask-Marshmallow and Marshmallow documentation entry points for file-upload handling. Done would be a clear documented answer about whether uploaded files can be validated this way, including the supported validation boundary or a stated limitation.

Written by the indexing model from the issue text.