Why you need to register an HPGL MIME type on your WEB server:

If your WEB server provides files that are in multiple formats, you must configure your server's Multiple Internet Mail Extensions (MIME) mapping to ensure your server will map the file type correctly when returning the file to remote browsers. If the MIME mapping on the server is not set up for a specific file type, browsers may not be able to retrieve the file.

Notice:
It seems that this step is not necessary if you use only Microsoft Internet Explorer (MSIE) as your WEB browser. MSIE does not need the MIME type registration on a WEB server from which it loads files. The MSIE can use only filename extensions to resolve which plug-in module to use. Each plugin module contains information about supported MIME types and about associated filename extensions to this MIME types. So if there is no conflict in filename extension between various plug-in modules, then the filename extension is a sufficient information for which plug-in to load. Both MSIE and Netscape use this method when loading a local file (e.g. C:\PLOTS\FILE.PLT). It is clear, because for local files there is not other information, only filename extension. However, the WEB server can provide another information to browser - MIME type (when this MIME type is registered on this WEB server). So when loading data from a WEB server, the Netscape requires to supply also the MIME type. It means that the desired MIME type must be registered on your WEB server when you are using Netscape (Navigator or Communicator) as your WEB browser.

How you can to register a MIME type on your WEB server:

At now I know MIME type registration process for these three WEB server software:

Note that in all instructions bellow I suppose registration of the application/vnd.hp-HPGL MIME type. The registration for any other MIME type is analogical.

MIME type registration instructions for the Alibaba WWW Server

If your WEB server is Alibaba WWW Server, do the following:
Run the Alibaba Administration tool and select the Mime types sheet. Here you will see a list of all already registered MIME types. You can easy add any other by typing it to the Mimetype / Subtype input box. Type the associated filename extension to the Extension input box. If you want to add more filename extensions for one MIME type, repeat the process for each extension with the same MIME type name.
Example:
Then you have registered application/vnd.hp-HPGL MIME type with the PLT extension.
If you want to add any other filename extension (e.g. HPG) for this MIME type:
Then you have registered application/vnd.hp-HPGL MIME type with the HPG extension.
And continue for all filename extensions which you want to associate to this MIME type.

After you are finished, don't forget to restart the Alibaba server.

MIME type registration instructions for the Microsoft Personal WEB Server

If your WEB server is Microsoft Personal WEB Server, do the following:
Go to the web server program directory, e.g.
C:\FrontPage Webs\Server
It is a directory where the web server program lies (Vhttpd32.exe).
In this directory you will find another directory named Conf, i.e.
C:\FrontPage Webs\Server\Conf
In this directory you will find a file named Mime.typ Open this file with Notepad.exe. You will see here a list of all already registered MIME types. You can add here any other MIME type along with all associated filename extensions.
Example:
Add to the Mime.typ file a line with the following content:
application/vnd.hp-HPGL plt hpg
Then you have registered application/vnd.hp-HPGL MIME type with the PLT and HPG extensions.
Note that all associated filename extensions are typed together on a single line along with a registered MIME type name.

After you are finished, don't forget to restart the Microsoft Personal WEB Server.

MIME type registration instructions for the Microsoft Internet Information Server

If your WEB server is Microsoft Internet Information Server, do the following:
Run the registry editor (regedit.exe or regedt32.exe) and go to the subkey
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\InetInfo\Parameters\MimeMap
You will see here a list of all already registered MIME types.
Note that each registered MIME type is defined as a string value, where all MIME type infromation is contained in the value name and the value itself is empty. The syntax for the MIME type value name is:

where mime type is a registered MIME type name and filename extension is an associated filename extension to this MIME type. I don't know yet what the gopher type mean, however I think that it is not important for WWW services (if you know anything about this please let me know).
Each MIME type value can contain only one filename extension. So if you want to associate more filename extensions to the same MIME type, you should define more MIME type value names with the same mime type and with different filename extension.
Example:
Add the
application/vnd.hp-HPGL,plt,,5
value name to the
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\InetInfo\Parameters\MimeMap
subkey. Then you have registered application/vnd.hp-HPGL MIME type with the PLT extension.
If you want to add any other filename extension (e.g. HPG) for this MIME type:
Add the
application/vnd.hp-HPGL,hpg,,5
value name to the
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\InetInfo\Parameters\MimeMap
subkey. Then you have registered application/vnd.hp-HPGL MIME type with the HPG extension.
And continue for all filename extensions which you want to associate to this MIME type.

After you are finished, don't forget to restart the Microsoft Internet Information Server.

Generally, see the Server MIME Mapping in Chapter 10 - Configuring Registry Entries in Internet Information Server HTML documentation.

I uderstand that using of registry editor for MIME type registration can be somewhat difficult for some less experienced users. It seems that Microsoft suposses that all IIS administrators are experienced users. If not, you can download here a registry file with predefined application/vnd.hp-HPGL MIME type for PLT and HPG filename extensions. You can easily edit this file as you wish and then force the registration by one of the following method:

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