How do I prompt a "Save As" dialog for an accepted mime type?

Often you will want a user to download / save a TXT, XLS, GIF, HTML or other file type that the browser would normally simply open and display within the browser (either from an HREF, a response.redirect, or a save target as). Here is one method I've used to force a prompt, in a file called download.asp: 
 

<%      Response.ContentType = "application/x-unknown" ' arbitrary      fn = "whatever.jpg"      FPath = "c:\" & fn      Response.AddHeader "Content-Disposition","attachment; filename=" & fn        Set adoStream = CreateObject("ADODB.Stream")      adoStream.Open()      adoStream.Type = 1      adoStream.LoadFromFile(FPath)      Response.BinaryWrite adoStream.Read()      adoStream.Close      Set adoStream = Nothing        Response.End  %>

 
And from the page where you want to launch this Save As dialog, you can do any method of redirection. Here are a few examples: 
 

<%      response.redirect("download.asp")  %>

 
 

<meta http-equiv='refresh' content='0;url=download.asp'>

 
 

<a href='download.asp'>Download whatever.jpg</a>

 
 

<script>  window.location.replace('download.asp');  </script>

 
Now keep in mind that the user can just decide to open it if they choose. 
 
One thing to keep in mind, is to avoid adding a custom cache-control header; the technique in Article #2022 could cause these downloads to fail. 
 
Some users have found that Internet Explorer prompts users with two open/save as dialogs. This is likely because you have sent the filename in both the ContentType and the AddHeader/Content-Disposition headers. To resolve the issue, use only the AddHeader variant. (See KB #238588 for more information.) 
 
Many people have complained that larger files simply do not work. You might see the following error messages from Internet Explorer: 
 

Unable to allocate required memory  --or  Not enough storage is available to complete this operation.  --or  Internet Explorer cannot download <file> from <server>.

 
You can try code like this instead, which sets response.buffer to false, sets a high server.scriptTimeout value, sets an explicit content-length header, and streams the file in chunks instead of all at once. Make sure you do not set an alternate character set using Response.CharSet. 
 

<%      Response.Buffer = False      Server.ScriptTimeout = 30000        Response.ContentType = "application/x-unknown" ' arbitrary      fn = "whatever.jpg"      FPath = "c:\" & fn      Response.AddHeader "Content-Disposition", "attachment; filename=" & fn        Set adoStream = CreateObject("ADODB.Stream")      chunk = 2048      adoStream.Open()      adoStream.Type = 1      adoStream.LoadFromFile(FPath)        iSz = adoStream.Size        Response.AddHeader "Content-Length", iSz        For i = 1 To iSz \ chunk          If Not Response.IsClientConnected Then Exit For          Response.BinaryWrite adoStream.Read(chunk)      Next        If iSz Mod chunk > 0 Then          If Response.IsClientConnected Then              Response.BinaryWrite adoStream.Read(iSz Mod chunk)          End If      End If        adoStream.Close      Set adoStream = Nothing        Response.End  %>

 
One other thing you might have to experiment with. To get the filename correct, try any of the following AddHeader lines... different versions of IE require a different command in order to override the filename. You might have to send a custom line depending on the version of Internet Explorer being used by the client. 
 

Response.AddHeader "Content-Disposition", "attachment; filename=" & fn    Response.AddHeader "Content-Disposition", "attachment; filename=" & fn & ";"    Response.AddHeader "Content-Disposition", "inline; filename=" & fn    Response.AddHeader "Content-Disposition", "inline; filename=" & fn & ";"    Response.AddHeader "Content-Disposition", "filename=" & fn    Response.AddHeader "Content-Disposition", "filename=" & fn & ";"

 
See Article #2129 for more information. Some Microsoft articles, for further reading: 
 
    KB #260519 
    HOWTO: Raise a "File Download" Dialog Box for a Known MIME Type 
 
    KB #262042 
    FIX: Content-Disposition: Attachment Saves Wrong Filename for Unknown Extension 
 
    KB #267991 
    FIX: "Content-Disposition: Attachment" Fails for Known Content Types 
 
    KB #279667 
    FIX: Content-Disposition Attachment Header Does Not Save File 
    (Also see Security Bulletin MS01-015.) 
 
    KB #816868 
    Error Message: "Internet Explorer Cannot Download a File"