Filestash has the plug built in.

Filestash puts a web interface in front of whatever storage you already have — S3, SFTP, WebDAV, a plain directory — and it carries a WOPI editor plugin in its core. Point that plugin at SumOffice and the files open in our engine. No patching, no fork.

What was measured

The round	`.xlsx` and `.docx` opened by a click in the Filestash file list, edited, saved back. The workbook took 13.5 s on a cold start, the document 1.8 s once the editor was warm.
What came back	The edit in place, the workbook’s Power Query binding intact, the document’s pictures intact.
Microsoft Excel and Word afterwards	Both open the returned files silently — no repair prompt. Measured on real files, opened in Microsoft Excel and Word on macOS after the round.
The one thing to know	Filestash has no file locking. The editor works without it, which means two people opening the same file are not held apart by the storage — if that matters for your team, a host with locks (Nextcloud, Infinite Scale, SharePoint) is the safer seat.
Limitation, on the Filestash side	Filestash’s own type table has no entry for `.xlsm`, `.xlsb` or `.docm`, so a macro file is handed to its text viewer instead of the office window — nothing the editor can change from its side. Through Filestash, count on `.xlsx` and `.docx`. If macros are the reason you are here, a host that routes them (Nextcloud, Infinite Scale, Group-Office, SharePoint) is the seat you want.
Two things that will stop you	`APPLICATION_URL` overrides the host stored in the admin console, and a request arriving on any other address is refused outright. And the local-directory backend asks for the admin password and an explicit path at the connection screen — an empty path is refused with no explanation on the page.

Setting it up

# the editors, behind your TLS address
git clone https://github.com/SumOfficeApp/sumoffice-docker && cd sumoffice-docker/nextcloud
cp env.example .env    # PUBLIC_URL=https://office.example.com
docker compose up -d

# Filestash → admin console → the WOPI editor plugin
# discovery URL:  https://office.example.com/hosting/discovery

Evaluation is free with no time limit; production is the per-application license. The other systems we measured →

Then

Check my Excel file — what your workbooks rely on, in seconds.
Tell us what you run behind Filestash — the storage decides how versions behave.