Recoll uses external applications to index some file types. You need to install them for the file types that you wish to have indexed (these are run-time optional dependencies. None is needed for building or running Recoll except for indexing their specific file type).
After an indexing pass, the commands that were found missing can be displayed from the recoll File menu. The list is stored in the missing text file inside the configuration directory.
A list of common file types which need external commands follows. Many of the filters need the iconv command, which is not always listed as a dependancy.
As of Recoll release 1.14, a number of XML-based formats that were handled by ad hoc filter code now use xsltproc, which usually comes with libxslt. These are: abiword, fb2 (ebooks), kword, openoffice, svg.
Openoffice: supported natively, but needs the unzip command to be installed.
Postscript: pstotext.
MS Word: antiword.
MS Excel and PowerPoint: catdoc.
MS Open XML (docx): needs xsltproc.
Wordperfect files: libwpd.
RTF: unrtf
TeX: Recoll uses the untex program. Your distribution may have a package for it. If it doesn't, there is a copy of the source on the Recoll web site, because the program has no obvious home. The filter can also work with detex and will use it if it is installed.
dvi: dvips
djvu: DjVuLibre
mp3, flac, ogg vorbis: Recoll releases before 1.13 use the id3info command from the id3lib package to extract mp3 tag information. (Some gcc versions after 4.4 may have trouble compiling id3lib. You can find a workaround here), metaflac (standard flac tools) for flac files, and ogginfo (vorbis tools) for ogg files. Releases 1.14 and later use a single Python filter based on mutagen for all audio file types.
Pictures: Recoll uses the Exiftool Perl package to extract tag information. Most image file formats are supported. Note that there may not be much interest in indexing the technical tags (image size, aperture, etc.). This is only of interest if you store personal tags or textual descriptions inside the image files.
chm: files in microsoft help format need Python and the pychm module (which needs chmlib).
ics: up to Recoll 1.13, iCalendar files need Python and the icalendar module. For newer versions, icalendar is not needed
zip: Zip archives need Python (and the standard zipfile module).
Text, HTML, mail folders, Openoffice and Scribus files are processed internally. Lyx is used to index Lyx files. Many filters need iconv and the standard sed and awk.