Is text/javascript or application/javascript correct?

The HTML specification now recommends text/javascript for scripts; both are widely accepted.

When do I need a charset?

Add charset=utf-8 to text-based types such as text/html and text/plain so the client decodes bytes correctly.

What if a type is unknown?

Use application/octet-stream, which tells the client to treat the body as opaque binary data.

MIME Type Lookup

Find the MIME type for a file extension, or the reverse.

MIME types (61)

Extension	MIME type
.aac	audio/aac
.avi	video/x-msvideo
.bin	application/octet-stream
.bmp	image/bmp
.bz2	application/x-bzip2
.csv	text/csv
.css	text/css
.doc	application/msword
.docx	application/vnd.openxmlformats-officedocument.wordprocessingml.document
.eot	application/vnd.ms-fontobject
.epub	application/epub+zip
.gif	image/gif
.gz	application/gzip
.htm	text/html
.html	text/html
.ico	image/vnd.microsoft.icon
.jar	application/java-archive
.jpeg	image/jpeg
.jpg	image/jpeg
.js	application/javascript
.json	application/json
.jsonld	application/ld+json
.mjs	application/javascript
.mp3	audio/mpeg
.mp4	video/mp4
.mpeg	video/mpeg
.oga	audio/ogg
.ogv	video/ogg
.ogx	application/ogg
.otf	font/otf
.pdf	application/pdf
.php	application/x-httpd-php
.png	image/png
.ppt	application/vnd.ms-powerpoint
.pptx	application/vnd.openxmlformats-officedocument.presentationml.presentation
.rar	application/vnd.rar
.rtf	application/rtf
.sh	application/x-sh
.svg	image/svg+xml
.swf	application/x-shockwave-flash
.tar	application/x-tar
.tif	image/tiff
.tiff	image/tiff
.ts	video/mp2t
.ttf	font/ttf
.txt	text/plain
.vsd	application/vnd.visio
.wav	audio/wav
.weba	audio/webm
.webm	video/webm
.webp	image/webp
.woff	font/woff
.woff2	font/woff2
.xhtml	application/xhtml+xml
.xls	application/vnd.ms-excel
.xlsx	application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
.xml	application/xml
.yaml	application/yaml
.yml	application/yaml
.zip	application/zip
.7z	application/x-7z-compressed

About the MIME Type Lookup

The Content-Type header tells a client how to interpret a response body. Get it wrong and browsers may download a page instead of rendering it, or refuse to run a script.

Search by extension such as .json or by media type such as image/ to find the right value, including whether a charset parameter is typically appropriate.