fix: file validators were editing the file fields content

[azmeuk]

When I was playing with the last flask-wtf I noticed that the file validators were editing the field.data since #556, leading to incompatibilities: on simple FileField my code was expecting a simple FileStorage and not a list of FileStorage. However the if not isinstance(field.data, list): field.data = [field.data] lines was setting a list even for simple FileField

@greyli what do you think?

[zorun]

FYI, we are also seeing failures in Ihatemoney. Here is an example code that now fails with flask-wtf 1.2.0, the form has a single FileField called file:

        form = ImportProjectForm()
        if form.validate():
            try:
                data = form.file.data
>               if data.mimetype == "application/json":
E               AttributeError: 'list' object has no attribute 'mimetype'

[guidoiaquinti]

Same issue here with a form with a simple FileField. On 1.2.0 is breaking with:

{"event": "ParamValidationError(\"Parameter validation failed:\\nInvalid type for parameter Body, value: [<FileStorage: 'passport.jpg' ('image/jpeg')>], type: <class 'list'>, valid types: <class 'bytes'>, <class 'bytearray'>, file-like object\")", "timestamp": "2023-10-02T06:12:56.024783Z", "thread": 281472627896800, "user_id": "qa7n0w6vku6ksld"}

[azmeuk]

The fix is in v1.2.1

[greyli]

@azmeuk Thanks for the quick fix! Sorry, I was on holiday last week.