Is Header('Content-Type:Text/Plain'); Necessary at All

Is it necessary to set a Content-Type in Node.js?

The Content-Type header is technically optional, but then you are leaving it up to the browser to essentially guess what type of content you are returning. Generally you should always specify a Content-Type if you know the type (which you probably do).

Content-Type:text/plain forces to download the file

I cannot reproduce this with this script:

<?php

  header('Content-Type:text/plain; charset=ISO-8859-15');
  echo "This is some text";

However, I can reproduce it with this:

<?php

  header('Content-Type:text/plain; charset=ISO-8859-15');
  echo "\x00This is some text";

Make sure that your content actually is plain ASCII text...

What are all the possible values for HTTP Content-Type header?

You can find every content types here:
http://www.iana.org/assignments/media-types/media-types.xhtml

The most common types are:

1. Type application:

    application/java-archive
    application/EDI-X12   
    application/EDIFACT   
    application/javascript   
    application/octet-stream   
    application/ogg   
    application/pdf  
    application/xhtml+xml   
    application/x-shockwave-flash    
    application/json  
    application/ld+json  
    application/xml   
    application/zip  
    application/x-www-form-urlencoded  
   

2. Type audio:

    audio/mpeg   
    audio/x-ms-wma   
    audio/vnd.rn-realaudio   
    audio/x-wav   
   

3. Type image:

    image/gif   
    image/jpeg   
    image/png   
    image/tiff    
    image/vnd.microsoft.icon    
    image/x-icon   
    image/vnd.djvu   
    image/svg+xml    
   

4. Type multipart:

    multipart/mixed    
    multipart/alternative   
    multipart/related (using by MHTML (HTML mail).)  
    multipart/form-data  
   

5. Type text:

    text/css    
    text/csv    
    text/html    
    text/javascript (obsolete)    
    text/plain    
    text/xml    
   

6. Type video:

    video/mpeg    
    video/mp4    
    video/quicktime    
    video/x-ms-wmv    
    video/x-msvideo    
    video/x-flv   
    video/webm   
   

7. Type vnd:

    application/vnd.android.package-archive
    application/vnd.oasis.opendocument.text    
    application/vnd.oasis.opendocument.spreadsheet  
    application/vnd.oasis.opendocument.presentation   
    application/vnd.oasis.opendocument.graphics   
    application/vnd.ms-excel    
    application/vnd.openxmlformats-officedocument.spreadsheetml.sheet   
    application/vnd.ms-powerpoint    
    application/vnd.openxmlformats-officedocument.presentationml.presentation    
    application/msword   
    application/vnd.openxmlformats-officedocument.wordprocessingml.document   
    application/vnd.mozilla.xul+xml   
   

How to set content-type when doing res.send()?

setHeader before sending: https://nodejs.org/api/http.html#http_response_setheader_name_value

res.setHeader('content-type', 'text/plain');
res.send(JSON.stringify({...}));

Do I need a content-type header for HTTP GET requests?

According to the RFC 7231 section 3.1.5.5:

A sender that generates a message containing a payload body SHOULD generate a Content-Type header field in that message unless the intended media type of the enclosed representation is unknown to the sender. If a Content-Type header field is not present, the recipient MAY either assume a media type of "application/octet-stream" ([RFC2046], Section 4.5.1) or examine the data to determine its type.

It means that the Content-Type HTTP header should be set only for PUT and POST requests.

Force Content-Type: text/plain to display in browser page

This sounds like an odd experiment, and you should probably send text/html instead, possibly with pre markup.

Anyway, octets 88 and 91 (hex.) denote control characters in ISO-8859-1, with no generally accepted assignment. If they are supposed to be printable characters, you should probably declare windows-1252 instead.

What content type to force download of text response?

To be on the safe side and ensure consistent behavior in all browsers, it's usually better to use both:

Content-Type: application/octet-stream
Content-Disposition: attachment;filename=\"My Text File.txt\"

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