implement [ so that it can keep "file" attribute

[romainfrancois]

We are about to release dplyr 1.0.8 and this package currently fails against it:

# telemac

<details>

* Version: 0.1.0
* GitHub: https://github.com/tpilz/telemac
* Source code: https://github.com/cran/telemac
* Date/Publication: 2021-02-19 10:10:05 UTC
* Number of recursive dependencies: 123

Run `cloud_details(, "telemac")` for more info

</details>

## Newly broken

*   checking re-building of vignette outputs ... WARNING
    ```
    Error(s) in re-building vignettes:
    --- re-building ‘t2d_basics.Rmd’ using rmarkdown
    Loading required package: sp
    Linking to GEOS 3.8.0, GDAL 3.0.4, PROJ 6.3.1; sf_use_s2() is TRUE
    ── Attaching packages ─────────────────────────────────────── tidyverse 1.3.1 ──
    ✔ ggplot2 3.3.5          ✔ purrr   0.3.4     
    ✔ tibble  3.1.6          ✔ dplyr   1.0.7.9000
    ✔ tidyr   1.1.4          ✔ stringr 1.4.0     
    ✔ readr   2.1.1          ✔ forcats 0.5.1     
    ── Conflicts ────────────────────────────────────────── tidyverse_conflicts() ──
    ...
    ✖ dplyr::filter() masks stats::filter()
    ✖ dplyr::lag()    masks stats::lag()
    Linking to GEOS 3.8.0, GDAL 3.0.4, PROJ 6.3.1; sf_use_s2() is TRUE
    --- finished re-building ‘t2d_rainfall_runoff.Rmd’
    
    SUMMARY: processing the following file failed:
      ‘t2d_basics.Rmd’
    
    Error: Vignette re-building failed.
    Execution halted
    ```

## In both

*   checking installed package size ... NOTE
    ```
      installed size is  8.3Mb
      sub-directories of 1Mb or more:
        doc       3.3Mb
        libs      3.4Mb
        telemac   1.0Mb
    ```

*   checking LazyData ... NOTE
    ```
      'LazyData' is specified without a 'data' directory
    ```

This is because dplyr no longer automatically keeps the attributes, but assumes that [ will do it appropriately for the class: https://github.com/tidyverse/dplyr/blob/0d8fb82362e348dade8f94a8e8dd24fa2eb660c9/R/generics.R#L242

cc @DavisVaughan

[tpilz]

Thanks. Will push it to CRAN next week.