Specifies file types which are allowed to be uploaded.
Possible Values: any valid Internet Media Type
This property defines a list of allowed content types. When no value is given for the property, files of any type can be uploaded. If value(s) are given, only files of the specified type(s) can be uploaded. The file type should be given as a valid Internet Media Type as defined by the IANA here:
To add multiple types, right click the property name and select 'Add Another Allowed Type'.
How Content-Type is Determined
When a file is uploaded via the File Upload and File Upload DND widgets the web browser determines the Content-Type value based on the file extension (the last few letters of the file name following the period). For example, csv, xlsx, jpg, and txt typically have Content-Type values of "text/csv", "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet", "image/jpeg", and "text/plain", respectively. The browser determines the Content-Type value from the client's operating system.
In Windows clients, the Registry is what maps the file extension to the Content-Type. For example, the HKEY_LOCAL_MACHINE\SOFTWARE\Classes\.png key maps the ".png" file extension to a Content-Type of "image/png", by default.
Note that these Registry values can change. Thus, the mappings are not necessarily the same for every client. In fact, the "csv" file type can be "text/csv" if a client desktop does not have Microsoft Office installed, whereas a client with Microsoft Office installed can have "csv" mapped to "application/vnd.ms-excel". If you wanted to permit only "csv" files to be uploaded, then to support both clients with and without Microsoft Office you would need to add both "text/csv" and "application/vnd.ms-excel" to the "allowed type" property.