Read .csv files and convert the rows into dict for looping
Welcome to CR!

As others recommended, you need to be more clear. I can give you some stylistic critique.

I like that you are using type hinting in the parameters. I recommend you also hint the return types for your functions like:

def _trim_row(self, row: dict) -&gt; list:
            trimmed_row = dict.fromkeys(self.FIELDS, None)
            for field in self.FIELDS:
                if field in row:
                     trimmed_row[field] = row[field]
            return trimmed_row


Now, more importantly I recommend you create strong documentation for your code so that we can understand the idea behind it, what bugs it has, as well as its efficiency failures (you can use codetags such as # BUG: or # FIXME:).

For your convenience, you can also edit your code in an IDE that enables pylinting, where you can get suggestions about how stylistically good your code is, I recommend VS Code with the Python and autoDocstring extensions. 

By putting your code in VS Code, I got the following suggestions for improvements:

""" [INSERT MODULE DESCRIPTION] """

import os
from csv import DictReader


class CSVHandler:
    """[INSERT CLASS DESCRIPTION]
    """
    DELIMIT = ','  # delimiter for csv
    FIELDS = set(list(['foo', 'bar']))  # fields that I want

    def __init__(self, file_path: str):
        """[summary]

        Arguments:
            file_path {str} -- [description]
        """
        self.file_path = file_path
        self.file_obj = None
        self.reader = DictReader([])

    def read_file(self):
        """[summary]

        Returns:
            [type] -- [description]
        """
        if self._is_file_ok:
            try:
                self.file_obj = open(self.file_path, newline='')
                self.reader = DictReader(self.file_obj)
                unmatched = self._is_fields_ok
                if isinstance(unmatched, set):
                    print(f"Warning : field set's unmatched! {unmatched}")
            except IOError:
                print(f'Unable to open/read file : "{self.file_path}"')
        else:
            print(f'Invalid file : "{self.file_path}"')
        return self

    @property
    def _is_file_ok(self):
        """[summary]

        Returns:
            [type] -- [description]
        """
        if os.path.isfile(self.file_path):
            if os.path.exists(self.file_path):
                if self.file_path.endswith('.csv'):
                    return True
        return False

    @property
    def _is_fields_ok(self):
        """[summary]

        Returns:
            [type] -- [description]
        """
        if self.reader or not self.FIELDS:
            return self.FIELDS - set(self.reader.fieldnames)
        return False

    def _trim_row(self, row: dict):
        """[summary]

        Arguments:
            row {dict} -- [description]

        Returns:
            [type] -- [description]
        """
        trimmed_row = dict.fromkeys(self.FIELDS, None)
        for field in self.FIELDS:
            if field in row:
                trimmed_row[field] = row[field]
        return trimmed_row

    def __enter__(self):
        return self

    def __iter__(self):
        return (self._trim_row(dict(row)) for row in self.reader)

    def __next__(self):
        return self._trim_row(dict(next(self.reader)))

    def __len__(self):
        return len(list(self.reader))

    def __exit__(self, exc_type, exc_val, exc_tb):
        if self.file_obj is not None:
            self.file_obj.close()


if __name__ == '__main__':
    with CSVHandler('file_name.csv').read_file() as csv_h:
        for rows in csv_h:
            pass


Another suggestion that I can give you is to know what you are importing and import just what you need. So, import os should be replaced with from os import path and remove os. from if os.path.isfile(self.file_path): and if os.path.exists(self.file_path):.

Following this, you will understand your code better, and you will be able to ask more specific questions about it.