Timeout uploading large files

**Describe the bug**
When uploading a large media file to a new post, the upload repeatedly times out after 60s.
As it's a big file (~50MB), I'd expect the upload to take longer than 60s, but the CMSs repeatedly aborts and retries after 60s.
It looks like this is intentional, perhaps caused by #3649?

Console output below:
```
Pausing requests for 4 seconds due to fetch failures: Request timed out after 60 seconds
cms.js:118591 Done pausing requests
cms.js:118589 Pausing requests for 9 seconds due to fetch failures: Request timed out after 60 seconds
cms.js:118591 Done pausing requests
cms.js:118589 Pausing requests for 16 seconds due to fetch failures: Request timed out after 60 seconds
react_devtools_backend.js:2450 leave called too many times.
cms.js:118591 Done pausing requests
cms.js:118589 Pausing requests for 25 seconds due to fetch failures: Request timed out after 60 seconds
react_devtools_backend.js:2450 leave called too many times.
cms.js:118591 Done pausing requests
react_devtools_backend.js:2450 API_ERROR: Request timed out after 60 seconds
at new t (cms.js:53893:25)
at i._createSuperInternal (cms.js:796:283)
at new i (cms.js:53886:27)
at E.handleRequestError (cms.js:54745:17)
at E._callee69$ (cms.js:54804:61)
at tryCatch (cms.js:223192:40)
at Generator.invoke [as _invoke] (cms.js:223421:22)
at Generator.prototype. [as throw] (cms.js:223244:21)
at asyncGeneratorStep (cms.js:223885:24)
at _throw (cms.js:223911:9)

cms.js:223888 Uncaught (in promise) {type: "ENTRY_PERSIST_FAILURE", error: "Failed to persist entry", payload: {…}}
```

**To Reproduce**
1. Go to the CMS, add a new content entry, of a type with a "file" field.
2. Click "choose a file", then "upload" and pick a large file.
3. Save the entry, wait a few minutes.
4. See error.

**Expected behavior**
Saving should eventually succeed.

**Applicable Versions:**
- netlify-cms-app: 2.13.1
- netlify-cms-core: 2.33.1
- netlify-cms: 2.10.66
- Git provider: GitHub
- OS: Windows 10
- Browser version chrome 86

**CMS configuration**
[config.yml](https://github.com/FTWinston/bvp/blob/master/static/admin/config.yml)

**Additional context**
If [this timeout](https://github.com/netlify/netlify-cms/pull/3649/files#diff-09e74c3942d1da38115b615f80c9b97f07a86d794548fe465b94e4db4148c7c3R13) were configurable, or could be disabled when uploading files, that would seem to me to prevent this problem.

Alternatively, if this is simply a much larger file than I ought to be storing in the CMS, then by all means tell me that.

Thanks in advance!