By default, Reflection FTP uses "Smart" or "automatic" selection for the transfer: this uses a table of associations for each file type together with a preferred transfer method. Sometimes, it does not have the appropriate method set for a particular type.
Using the [Help] button, you can look through the various options available: in general, if you are not sure which transfer method is appropriate, use the "Binary" method. It may not be most efficient, but generally will work fine.
Here is the relevant Help text for the binary selection:
