fix: uploadButton data-max-filesize attribute is not passed to file-uploader

[fabien-michel]

The attribute exists on the upload button, but it is not read and used.

This PR fix upload-button.js to read the value tranform from MiB to bytes and pass the param to FileUploaderBasic object.

By the way, the library used is more than 10 years old of age (2011 ?) and is not even maintained since 2018 (repo archived)
Also it is strange to have 2 distinct upload mechanisms, one for dropzone and one for upload button.

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[fsbraun]

@fabien-michel Did you test this?

[fabien-michel]

Yes but maybe I miss something. What do you mean ?

[fsbraun]

I just wanted to verify that with the previous PR this actually makes the upload limit work.

[fabien-michel]

Ok sorry I understand I should have give more informations.

In order to understand how to set up the uploader, I've found the uncompressed file uploader JS file here : https://github.com/FineUploader/fine-uploader/blob/83e14f51b2421b80c14cd0c28668171a6777a154/client/fileuploader.js#L272

The file uploader display a popup when file exceed the max. (weirdly displayed in GiB)

Example for a limit to 100MiB (FILER_UPLOADER_MAX_FILE_SIZE=100)