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Incorrect or missing file Content-type in RadAsyncUpload


When uploading a file, the content type is null or application/octet-stream.


When uploading a .pages file from MAC: The content type of the uploaded file is "application/x-iwork-pages-sffpages".

When uploading a .pages file from Windows: The content type of the uploaded file is null or the default for binary files "application/octet-stream".


The explanation for this behavior is the way the browsers decide the content type of the file.**

  1. The browser determines the media type of the file based on a hardcoded list of available file extensions and content types - MDN web docs - MIME types.

  2. If there are no matches, the browser looks in the registry files of the computer.

  3. If no content-type is available for the file extension type, the browser looks in a secondary hard-coded list.

  4. Finally, if there are no matched anywhere, the browser puts the default content type, which for a binary file is "application/octet-stream".

The RadAsyncUpload control sets the content type for unknown file extensions to null as the JavaScript drop event does not provide any content type. Here is a sample example that could be used to check the scenario. 

function OnClientFileDropped(sender,args) {
    var ev = args.get_originalDropEvent();
    var contentType = ev.originalEvent.dataTransfer.items[0].getAsFile().type;
    console.log("Content type is: " + contentType);
function drop_handler(ev) {
    var contentType = ev.dataTransfer.items[0].getAsFile().type;
    console.log("Content type is: " + contentType);
<telerik:RadAsyncUpload ID="RadAsync_FileUpload" RenderMode="Lightweight" OnFileUploaded="RadAsyncUpload1_FileUploaded" OnClientFileDropped="OnClientFileDropped" runat="server" OnFileUploaded=></telerik:RadAsyncUpload>
<input type="file" id="myFile" name="myFile" ondrop="drop_handler(event);" />

The results when tested on Windows for both the RadAsyncUpload and the input are as follows:

  • When uploading a .jpg file: "Content type is: image/jpeg"

  • When uploading a .pages file: "Content type is: "

There are some resources related to the decision of the content type from the browser:


One possible solution for this is to check the content type in the FileUploaded event, and if it is null or "application/octet-stream" to define it manually based on the file extension from a predefined list with known content types.

protected void RadAsyncUpload1_FileUploaded(object sender, FileUploadedEventArgs e)
    string contentType = e.File.ContentType;
    if (string.IsNullOrEmpty(contentType))
        contentType = "application/octet-stream";
        // or other based on e.File.GetExtension()
        //if (e.File.GetExtension() == ".pages")
        //    contentType = "application/x-iwork-pages-sffpages";
Protected Sub RadAsyncUpload1_FileUploaded(sender As Object, e As FileUploadedEventArgs)
    Dim contentType As String = e.File.ContentType
    If String.IsNullOrEmpty(contentType) Then
        contentType = "application/octet-stream"
        ' or other based on e.File.GetExtension()
        'If e.File.GetExtension() = ".pages" Then
        '   contentType = "application/x-iwork-pages-sffpages"
        'End If
    End If
End Sub
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