Adds Standard CSV Mime Type

[SoftWyer]

On Android, the CSV mime type from getMimeTypeFromExtension() returns "text/comma-separated-values" which is non-standard and doesn't filter CSV files in Google Drive.

This change adds the standard "text/csv" mime type when the csv extension is specified.

We did think about adding the ability to specify explicit mime types from the API, but this would be a bigger change when this only seems to be an Android issue.

see standards docs
see Android code

[miguelpruivo]

Hi, thank you for your PR but the problem here is that iOS doesn't support it and may result in some devs assuming that is working on both and then finding that iOS doesn't support this kind of filter and then later report yet another issue.

Every filter should be supported from all OS to avoid unnecessary results.

[SoftWyer]

Hi @miguelpruivo, perhaps you've misunderstood the change. Filtering the file selection by CSV is already supported by both Android and iOS. We can see files with these extensions when using file-picker on these devices.

The problem is that Android uses a non-standard CSV mime type extension. This means that it can filter on-device CSV files, but cannot filter CSV files using any cloud service (e.g. Google Drive) as these use the standard mime type. This change just ensures that CSV filtering works on Android as expected regardless of file system. I tangentially mentioned the CSV problem in this issue.

If you'd rather not include this PR, that it OK with us. We needed Android CSV filtering to work consistently in our app so thought we'd give those changes back to the project 😃

[miguelpruivo]

Ok, looks like a nice addition. Could you please rebase to the latest version and update changelog/pubspec accordingly? Thanks!

[SoftWyer]

Latest changes merged, should be good to go 👍