Allow use of file extensions in gr.File in iOS

[hannahblair]

Description

This one is a yet another annoying iOS issue. We accept file extensions (alongside image/* etc, this is fine) in the file_types attribute in gr.File, but in iOS these file extensions won't actually work, and will grey out all the options. We could process each and every possible file extension that might be used and get the corresponding mime type but I don't like this approach.

I've opted to validate the file after the file has been selected, showing an error with the expected file type. I'd hope that the expected file types would be displayed either in the info text by the user or some point soon in the actual upload area which I think we've discussed before (I should raise an issue for this, it would improve UX).

Here's how the overall logic looks in the File component now:

filetype Input	accept_file_types	File Picker Behavior	Post-Upload Validation
['.csv', '.doc']	".csv, .doc"	Allows all files	Yes, filters to .csv and .doc
'.csv'	".csv"	Allows all files	Yes, filters to .csv
'file/*'	"*"	Allows all files	No
['image/', 'text/']	"image/, text/"	Shows image and text files	No
'image/*'	"image/*"	Shows image files	No

Test example:

Uploading a .las file when file_types=['.las']

Closes: #7930

Caniuse reference

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Allow use of file extensions in gr.File in iOS

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[dawoodkhan82]

The code lgtm! Thanks for adding this. I couldn't test on iOS, but trusting it works.I think having the error for now should be good enough until we refactor the file upload Ui to include more context.