[TYPO3-Solr] limit dam indexing to specific file-types

[Schwarzenberg]

hi list,

i have some questions regarding indexing ext:dam records.
i use ext:solr v2.2.2 and already managed it to get dam contents indexed. this is the corresponding 
typoscript configuration:

plugin.tx_solr.index.queue {
        dam = 1
        dam {

                 additionalPageIds = {$MEDIA_UID}
                 table = tx_dam

                 fields {

                         title = title
                         content = description

                         filetype_stringS = file_type
                         filename_stringS = file_name
                         filepath_stringS = file_path

                         sortTitle_stringS = title

                         keywords = SOLR_MULTIVALUE
                         keywords.field = keywords

                         keywords_stringM = SOLR_MULTIVALUE
                         keywords_stringM.field = keywords

                         url = TEXT
                         url {
                                 typolink.parameter.dataWrap = {field:file_path}{field:file_name}
                                 typolink.useCacheHash = 1
                                 typolink.returnLast = url
                         }

                 }

         }
}

the unwanted side-effect (for me :-) ) now is, that images also get indexed. so, if i search for 
"wlan" for example, i also get an image called "wlan.jpg" in my search results,  which i do not 
want. is it possible to limit the indexing to specific files, for example only to pdf-Documents?

regards, heiko