This avoids appending extra extensions

[temehi]

A prefix can represent files with various extensions and/or mimetypes. As a result one can not specify in advance supported file types for an input/output prefix ports. Which means supported files are any files with that prefix. Something like "prefix.* " .... This fix avoids extra extension taken from the valid types and enables us to use ".*" as a valid type and closes the issue I reported issue #115.

Before it looked like these.

/path/to/prefix.ext.ext1 
/path/to/prefix.ext.ext2 
/path/to/prefix.ext.ext3 ... 

now

/path/to/prefix.ext1 
/path/to/prefix.ext2 
/path/to/prefix.ext3 ...