[custom-handler] Allow specifying a custom file reader handler

[kensnyder]

[stargazing-dino]

File reading is not my thing but would this line

const blob = file.getAsFile ? file.getAsFile() : file;

be better served before the fileReader callback is checked for and called? That way we can be semi sure a File is returned in the callback instead of an Item?

[kensnyder]

@Nolence Let me update the PR.

[kensnyder]

@Nolence have a look now.

[zachzoup]

Is this going to be merged? Doesn't seem to be any issues but it's been almost three months.