perf(files): Cache `getContents` function used for uploader

perf(files): Cache `getContents` function used for uploader

Instead of trigger a PROPFIND for every new-menu entry clicks,
or conflict handling of uploads, we can just use the cached content from the file store.
If we do not have any cache entry we fetch new, but otherwise this is not needed.

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>