Add relative path to upload if webkitdirectory attribute used

[easink]

Now you can do this:
<%= live_file_input @uploads.files, webkitdirectory: true %>
and get %UploadEntry{} with a relative path.

One of the drawbacks of this implementation is that you cannot drag and drop folders, you than need some more javascript to make it work. (https://stackoverflow.com/questions/3590058/does-html5-allow-drag-drop-upload-of-folders-or-a-folder-tree)
But I think this is an important feature as is.

Another drawback is that this not supported on most mobile phones, but should not be a problem anyway.
(https://caniuse.com/?search=webkitdirectory)

There is a reported issue on this: #1482

[mcrumm]

Hi @easink! Thanks for the PR. Generally we avoid adding support for non-standard features. Using the client file name as the default path name is also a concern because it is unsanitized input.

I will leave this open for now though so other can weigh in :)

[josevalim]

Given the functionality is mostly copying the relative path, I would be ok with supporting it and we can fallback to reading other attributes if supported in the future.

[rsmidt]

I would love to have this in LiveView. Is there something we can do to allow the PR to be merged?

[chrismccord]

Thank you @easink ! Can you update the PR to make the field relative_path and make it null rather than falling back to name when it's not present? Thanks!

[easink]

I rebased it to phoenix_live_view/master and github went bananas.

[easink]

Does anybody have any trouble with this? Works for me.

[chrismccord]

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