A common issue with converters is running out of memory. This is commonly reported like:
Failed to convert type unknown for [PATH TO FILE]: \ [CONVERTER_MEMORY_LIMIT] Memory limit of 5000000 bytes exceeded.
When this error is reported, you'll want to increase the converter memory, CPU and timeout limits located on the collection's Configuration > Crawler tab, in the Global Settings section's Converting sub-section. Increasing these values and attempting to convert again will usually result in a successful conversion. Be careful not to increase the limits beyond the resources that are available on the system.