fix curl command upload error

[moononournation]

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[moononournation]

The original ESP32 FSBrowser can use curl upload SPIFFS files:

for file in `ls -A1`; do curl -F "file=@$PWD/$file" fsbrowserPlus.local/edit; done

This pull request help to prepend "/" to the filename for fulfilling the SPIFFS filename format and return HTTP 200 after finished. (avoiding curl received HTTP 500 at last)