Missing charset in Content-Type header for workers static assets when deployed

### What versions & operating system are you using?

Wrangler 4.20.0, Node v22.16.0, Fedora 42 Workstation

### Please provide a link to a minimal reproduction

https://github.com/azrdn/azrd.dev/tree/cicada

### Describe the Bug

On local wrangler development, the charset in header exists, but not after deployed
Doesn't match default pages behaviour, and results in garbled text when viewed in browser

### Please provide any relevant error logs

```
azrd@fedora in ~
↪ curl -I http://localhost:8787/robots.txt
HTTP/1.1 200 OK
Content-Type: text/plain; charset=utf-8
Cache-Control: public, max-age=0, must-revalidate
ETag: "63ac36e7ea7c3b25b65a9a51ce23370d"
CF-Cache-Status: HIT
content-security-policy: object-src 'none'; frame-ancestors 'none'; upgrade-insecure-requests
cross-origin-opener-policy: same-origin
referrer-policy: no-referrer
x-robots-tag: noarchive

azrd@fedora in ~
↪ curl -I https://web.azrd.workers.dev/robots.txt
HTTP/2 200
date: Fri, 13 Jun 2025 09:26:31 GMT
content-type: text/plain
cf-ray: 94f08bfc091b46f7-SIN
cf-cache-status: HIT
cache-control: public, max-age=0, must-revalidate
etag: "f78716248fc6d4316d80b767b0a4867f"
content-security-policy: object-src 'none'; frame-ancestors 'none'; upgrade-insecure-requests
cross-origin-opener-policy: same-origin
referrer-policy: no-referrer
x-robots-tag: noarchive
report-to: {"endpoints":[{"url":"https:\/\/a.nel.cloudflare.com\/report\/v4?s=YxZzJSJ7scgRVqxvrvxeXQf%2BQJZC7WqbnTgxLrSKtmaqvVREJ0WVmVgFFHfrO7TUXK6HX7oc3Z4hUQKKLeV%2BL4z8o1w3%2BBWK92Gl6nC9NXXPIoU60l8vZ9uAIVzVSrRnfk1h%2Bd0w9w%3D%3D"}],"group":"cf-nel","max_age":604800}
nel: {"success_fraction":0,"report_to":"cf-nel","max_age":604800}
server: cloudflare
alt-svc: h3=":443"; ma=86400
server-timing: cfL4;desc="?proto=TCP&rtt=38699&min_rtt=36550&rtt_var=9700&sent=10&recv=15&lost=0&retrans=0&sent_bytes=3400&recv_bytes=772&delivery_rate=95366&cwnd=253&unsent_bytes=0&cid=d59f40f32757bebf&ts=93&x=0"

azrd@fedora in ~
↪ curl -I https://azrd.pages.dev/robots.txt
HTTP/2 200
date: Fri, 13 Jun 2025 09:26:40 GMT
content-type: text/plain; charset=utf-8
cf-ray: 94f08c35dd8b407f-SIN
access-control-allow-origin: *
cache-control: public, max-age=0, must-revalidate
etag: "f78716248fc6d4316d80b767b0a4867f"
content-security-policy: object-src 'none'; frame-ancestors 'none'; upgrade-insecure-requests
cross-origin-opener-policy: same-origin
referrer-policy: no-referrer
x-content-type-options: nosniff
x-robots-tag: noarchive
report-to: {"endpoints":[{"url":"https:\/\/a.nel.cloudflare.com\/report\/v4?s=AAOjl3OHhmSVZxGVaVnt%2FyTya2ah8215377LY2J1GFvPKtJ3%2BYnuOQnI%2FS%2BexYpVpe6TzVWzuzoOIlTCLxfLc%2FkbSCOE3JRsUIvRy9vMHAPFSj3cj3zmX5ip5xivsN%2FfjA%3D%3D"}],"group":"cf-nel","max_age":604800}
nel: {"success_fraction":0,"report_to":"cf-nel","max_age":604800}
server: cloudflare
alt-svc: h3=":443"; ma=86400
server-timing: cfL4;desc="?proto=TCP&rtt=36290&min_rtt=33302&rtt_var=9988&sent=7&recv=9&lost=0&retrans=0&sent_bytes=3398&recv_bytes=768&delivery_rate=92638&cwnd=253&unsent_bytes=0&cid=8eb864fadabe4998&ts=444&x=0"
```