Media: cannot upload new images on a site using 75% of the allotted upload limit

Steps to reproduce:

1. Create a WordPress.com site and add a Personal Plan.
2. Upload ~4.5 GB of images.
3. On the web, check that the Media > Add New page says something like: "You are currently using 4.5 GB out of 6.0 GB upload limit (75%)."
4. On the app, create a new post and try to upload an image.
5. If the upload fails, check the app logs to see if you can find a message similar to: "Not enough disk space available to upload".

Result: a user reported being unable to upload new images on a site and there is a line in the app logs that starts with "Error uploading media" and says there is not enough disk space available even though they have only used 75% of the allotted storage space for their plan.

Problem description from the user:

I’m trying to upload pictures on my blog and for some reasons it doesn't work. No explanation is given and I was wondering whether you could help me with that.

App log says:

Pas assez d’espace disque disponible pour téléverser
Utilisation de 100 % sur 6 Go sur votre site

Translated with Google:

Not enough disk space available to upload
Use of 100% on 6 GB on your site

But /wp-admin/media-new.php says:

You are currently using 4.5 GB out of 6.0 GB upload limit (75%).

• Site type: WordPress.com Personal
• Device: iPhone 7 iOS 12.3.1
• Free space: 8.83 GB
• App version: 12.7

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Start by reproducing the upload from a WordPress.com Personal site using the app logs and compare the app's reported storage with /wp-admin/media-new.php. Done means a site using about 75% of its 6 GB limit can upload an image and the app reports storage consistently with the web interface.