Make Mojo::Asset::File writable to the existed files.

After debugging for days, I figured it out finally and fixed my code.
This patch is to make the Mojo::Asset::File more flexible, it gives the write permission to the module when handling uploading/downloading files.


Thank you very much for supporting me, my html5 large files "breakpoint resume" developed using the latest Mojolicous finished.

My project addresses: https://github.com/iakuf/mojolicious-stream-upload