How can I upcycle large file cabinets?

You can use them for storage in your garage or turn them into planters:

https://www.hometalk.com/diy/home-office/furniture/painting-an-old-file-cabinet-to-make-a-large-colorful-planter-9590726

Lay them in their side for sports equipment or yard tools organizing. At the very least, recycle them.

http://www.greenecoservices.com/23-ways-to-reuse-file-cabinets/

https://www.bhg.com/decorating/makeovers/furniture-makeovers/how-to-repurpose-filing-cabinets/

Here is a great idea from Hometalk https://www.hometalk.com/diy/home-office/furniture/anthropologie-hack-flea-market-flip-evidence-cabinet-reveal-30036279

A few more idea possibilities

https://thecottagemarket.com/14-awesome-upcycled-repurposed-filing-cabinets-will-wow/

Some diy-er's have removed the drawers, placed the cabinets with the openings to the top to use for storage of rakes, hoes,garden equipment in their storage sheds or garages. Then use the drawers as outdoor planters. You can use them "as-is" for good storage in a garage or when all else fails, someone might want them if you place them at roadside.

Here are some more ideas! https://www.hometalk.com/search/all?filter=file%20cabinet

Use for Christmas decorations books

You can remove the drawers, turn the file cabinet sideways, spray paint, and create a storage bench. Add a cushion and put storage baskets where the drawers were.

Here's a fab transformation

https://www.hometalk.com/diy/home-office/furniture/learn-how-to-transform-an-ordinary-file-cabinet-into-extraordinary-29171132

I too have used filing cabinets for my garden. I placed the back side on the ground, removed the drawers, and filled with mulch, potting soil, and sand. The cucumbers, herbs, onions, tomatoes, and carrots grew great.

We used the drawers in our Chicken coop for nesting boxes.

One of my friends laid a filing cabinet on it's side, removed the drawers, painted it, and used it as a bench outside and used the opening where the drawers were to put garden tools in buckets, and such.