framework/validators/FileValidator.php
protected function validateMimeType($file)
    {
        $fileMimeType = FileHelper::getMimeType($file->tempName);

        foreach ($this->mimeTypes as $mimeType) {
            if ($mimeType === $fileMimeType) {
                return true;
            }

            if (strpos($mimeType, '*') !== false && preg_match($this->buildMimeTypeRegexp($mimeType), $fileMimeType)) {
                return true;
            }
        }

        return false;
    }
protected function validateMimeType($file)
    {
        $fileMimeType = FileHelper::getMimeType($file->tempName);

        foreach ($this->mimeTypes as $mimeType) {
            if ($mimeType === $fileMimeType) {
                return true;
            }

            if (strpos($mimeType, '*') !== false && preg_match($this->buildMimeTypeRegexp($mimeType), $fileMimeType)) {
                return true;
            }
        }

        return false;
    }
Checks the mimeType of the $file against the list in the [[mimeTypes]] property.

@param UploadedFile $file
@return bool whether the $file mimeType is allowed
@throws \yii\base\InvalidConfigException
@see mimeTypes
@since 2.0.8