The list of supported data types is available through the GdkDragContext, but this list is not checked before requesting that the source application convert the selection to the (often unsupported) type. One common case this hits is reading numDropItems from an nsIDragSession, as is done when constructing an nsDOMDataTransfer. (The data is requested in text/uri-list form, whether or not this is available.) Attachment 271701 [details] [diff] for bug 179658 addresses this (but apparently it has issues that need to be resolved).
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