Document that File.Select.file/files' Mime type argument might not be respected

[jjant]

Basically, the accept attribute doesn't really work that well (setting it to just image/png will still let iOS users pick any image/* from their gallery, and will let android users pick videos).

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I think the docs for these functions should do either or both of:

• Explain that it uses accept under the hood (I had to read the source to find that out)
• Explain that this is not a reliable way to filter file types