Fix FileInput.save

[Hoxbro]

Fixes #3578

[maximlt]

Thanks for the quick PR!!! Could you also add this test?

import pathlib

...

def test_file_input_save_one_file(document, comm, tmpdir):
    file_input = FileInput(accept='.txt')

    widget = file_input.get_root(document, comm=comm)

    assert isinstance(widget, BkFileInput)

    file_input._process_events({'mime_type': 'text/plain', 'value': 'U29tZSB0ZXh0Cg==', 'filename': 'testfile'})

    fpath = pathlib.Path(tmpdir) / 'out.txt'
    file_input.save(str(fpath))

    assert fpath.exists()
    content = fpath.read_text()
    assert content == 'Some text\n'

[maximlt]

Thanks for the changes! The test suite is still broken due to the latest release of HoloViews, we'll have to re-run the tests when this is fixed.

[philippjfr]

Thanks @Hoxbro, great and quick work as always!