using `splitext` instead of `split`

[t-bltg]

One should use splitext instead of split, since using split fails if a . is present in the filename (e.g. hidden directory on linux):

julia> split("/foo/bar/.hidden/fig.jpg", ".")[2]
"hidden/fig"
julia> splitext("/foo/bar/.hidden/fig.jpg")[2]
".jpg"

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (403d972) 90.21% compared to head (476fb7a) 90.21%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #23 +/- ## ======================================= Coverage 90.21% 90.21% ======================================= Files 5 5 Lines 184 184 ======================================= Hits 166 166 Misses 18 18 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaImages/ExifViewer.jl/pull/23?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaImages) | Coverage Δ | | |---|---|---| | [src/utils.jl](https://app.codecov.io/gh/JuliaImages/ExifViewer.jl/pull/23?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaImages#diff-c3JjL3V0aWxzLmps) | `55.55% <ø> (ø)` | | | [src/write.jl](https://app.codecov.io/gh/JuliaImages/ExifViewer.jl/pull/23?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaImages#diff-c3JjL3dyaXRlLmps) | `88.88% <100.00%> (ø)` | |

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[ashwani-rathee]

@t-bltg thanks for your first contribution to ExifViewer.jl and first ever contribution to any of my packages actually!!

Writing tags needs a lot more work, let's merge this!