It is easy to add a single file upload button in the MDriven Designer, but to handle multiple files you'll have to create a component.

Model for this example

The model for the example is a project with some documents. Each document holds the file (a blob) and a filename (a string). The goal is to make it possible to choose several documents in the file browser and upload all of them.

Setting up the ViewModel

• Create a view model. It should have a root, which in this case should be a project. The project holds the list / container of documents. We will need the ID of the view model class.
• Create a variable (here named vUploadFileTarget). It should be of a type that could store a file (blob). In this example we choose the Document type. This variable will be used to temporary store each file since we are limited to one single upload at a time.

• Add a generic view model column named AttachmentUploadTarget with the value of the file part of your variable (here vUploadFileTarget.File). The name here will be used in the component code. We create a view model column to be able to handle it in the component client code.
• Add a generic view model column named AttachmentUploadTarget_FileName with the value of the string part of your variable (here vUploadFileTarget.FileName). The _FileName suffix is important since there is a MDriven "magic" that, when uploading a file, will look for a column with the same name as referenced but with a "_FileName" suffix and if such a column exists it will assign the filename to it. Thus, this part is optional, but if you skip it you will lose the name of the file.
• Set Visible Expression to false for these columns. They are only for temporary storage.

• Add a generic view model column, with a name of your choice, for uploading (it will be a button).
• Check the Content override checkbox. This is the column that will use our own component.
• Click the TV and Attributes button, which will open a dialogue where you can set up the usage of your own external component.