diff --git a/src/adapters.rs b/src/adapters.rs
index 8584a91..bf382fb 100644
--- a/src/adapters.rs
+++ b/src/adapters.rs
@@ -1,6 +1,6 @@
 pub mod custom;
 pub mod decompress;
-//pub mod ffmpeg;
+pub mod ffmpeg;
 pub mod fns;
 //pub mod pdfpages;
 pub mod poppler;
@@ -95,7 +95,7 @@ pub fn get_all_adapters(custom_adapters: Option<Vec<CustomAdapterConfig>>) -> Ad
     }
 
     let internal_adapters: Vec<Rc<dyn FileAdapter>> = vec![
-        //Rc::new(ffmpeg::FFmpegAdapter::new()),
+        Rc::new(ffmpeg::FFmpegAdapter::new()),
         //Rc::new(zip::ZipAdapter::new()),
         Rc::new(decompress::DecompressAdapter::new()),
         // Rc::new(tar::TarAdapter::new()),
diff --git a/src/adapters/custom.rs b/src/adapters/custom.rs
index 71d7c6c..94923e4 100644
--- a/src/adapters/custom.rs
+++ b/src/adapters/custom.rs
@@ -3,9 +3,12 @@ use super::{
     AdapterMeta, GetMetadata,
 };
 use crate::matching::{FastMatcher, SlowMatcher};
+use anyhow::{Context, Result};
 use lazy_static::lazy_static;
+use regex::{Captures, Regex};
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
+use std::path::Path;
 
 // mostly the same as AdapterMeta + SpawningFileAdapter
 #[derive(Debug, Deserialize, Serialize, JsonSchema, Default, PartialEq, Clone)]
@@ -115,17 +118,59 @@ impl GetMetadata for CustomSpawningFileAdapter {
         &self.meta
     }
 }
+fn arg_replacer(arg: &str, filepath_hint: &Path) -> Result<String> {
+    lazy_static::lazy_static! {
+        static ref ARG_REP: Regex = Regex::new(r"\{([a-z_]+)\}").unwrap();
+    }
+    let mut err = None;
+    let r = ARG_REP.replace_all(arg, |m: &Captures| -> String {
+        let idx = m.get(0).unwrap().range();
+        if arg.chars().nth(idx.start - 1) == Some('{') {
+            // skip
+            return m.get(0).unwrap().as_str().to_string();
+        }
+        if arg.chars().nth(idx.end + 1) == Some('}') {
+            // skip
+            return m.get(0).unwrap().as_str().to_string();
+        }
+        let key = m.get(1).unwrap().as_str();
+        if key == "file_extension" {
+            return filepath_hint
+                .extension()
+                .map(|e| e.to_string_lossy().to_string())
+                .unwrap_or("".to_string());
+        }
+        err = Some(anyhow::anyhow!(
+            "Unknown arg replacement key '{}' in '{}'",
+            key,
+            arg
+        ));
+        "".to_string()
+        //let
+    });
+    if let Some(err) = err {
+        Err(err)
+    } else {
+        Ok(r.to_string())
+    }
+}
 impl SpawningFileAdapterTrait for CustomSpawningFileAdapter {
     fn get_exe(&self) -> &str {
         &self.binary
     }
     fn command(
         &self,
-        _filepath_hint: &std::path::Path,
+        filepath_hint: &std::path::Path,
         mut command: std::process::Command,
-    ) -> std::process::Command {
-        command.args(&self.args);
-        command
+    ) -> Result<std::process::Command> {
+        command.args(
+            self.args
+                .iter()
+                .map(|arg| arg_replacer(arg, &filepath_hint))
+                .collect::<Result<Vec<_>>>()?,
+        );
+        log::debug!("running command {:?}", command);
+        Ok(command)
     }
 }
 impl CustomAdapterConfig {
diff --git a/src/adapters/ffmpeg.rs b/src/adapters/ffmpeg.rs
index e23751b..e3e09b7 100644
--- a/src/adapters/ffmpeg.rs
+++ b/src/adapters/ffmpeg.rs
@@ -2,9 +2,11 @@ use super::spawning::map_exe_error;
 use super::*;
 use anyhow::*;
 use lazy_static::lazy_static;
+use regex::Regex;
 use serde::{Deserialize, Serialize};
 use std::io::BufReader;
 use std::process::*;
+use writing::{WritingFileAdapter, WritingFileAdapterTrait};
 // todo:
 // maybe todo: read list of extensions from
 // ffmpeg -demuxers | tail -n+5 | awk '{print $2}' | while read demuxer; do echo MUX=$demuxer; ffmpeg -h demuxer=$demuxer | grep 'Common extensions'; done 2>/dev/null
@@ -26,12 +28,12 @@ lazy_static! {
     };
 }
 
-#[derive(Default)]
+#[derive(Default, Clone)]
 pub struct FFmpegAdapter;
 
 impl FFmpegAdapter {
-    pub fn new() -> FFmpegAdapter {
-        FFmpegAdapter
+    pub fn new() -> WritingFileAdapter {
+        WritingFileAdapter::new(Box::new(FFmpegAdapter))
     }
 }
 impl GetMetadata for FFmpegAdapter {
@@ -48,12 +50,16 @@ struct FFprobeOutput {
 struct FFprobeStream {
     codec_type: String, // video,audio,subtitle
 }
-impl FileAdapter for FFmpegAdapter {
-    fn adapt(&self, ai: AdaptInfo, _detection_reason: &SlowMatcher) -> Result<()> {
+impl WritingFileAdapterTrait for FFmpegAdapter {
+    fn adapt_write(
+        &self,
+        ai: AdaptInfo,
+        _detection_reason: &SlowMatcher,
+        oup: &mut dyn Write,
+    ) -> Result<()> {
         let AdaptInfo {
             is_real_file,
             filepath_hint,
-            oup,
             line_prefix,
             ..
         } = ai;
@@ -80,7 +86,7 @@ impl FileAdapter for FFmpegAdapter {
                     "stream=codec_type",
                 ])
                 .arg("-i")
-                .arg(inp_fname)
+                .arg(&inp_fname)
                 .output()
                 .map_err(spawn_fail)?;
             if !probe.status.success() {
@@ -107,7 +113,7 @@ impl FileAdapter for FFmpegAdapter {
                     //"-count_packets",
                 ])
                 .arg("-i")
-                .arg(inp_fname)
+                .arg(&inp_fname)
                 .stdout(Stdio::piped())
                 .spawn()?;
             for line in BufReader::new(probe.stdout.as_mut().unwrap()).lines() {
@@ -125,7 +131,7 @@ impl FileAdapter for FFmpegAdapter {
                 .arg("-loglevel")
                 .arg("panic")
                 .arg("-i")
-                .arg(inp_fname)
+                .arg(&inp_fname)
                 .arg("-f")
                 .arg("webvtt")
                 .arg("-");
diff --git a/src/adapters/fns.rs b/src/adapters/fns.rs
index 4f6e9e8..47cf93c 100644
--- a/src/adapters/fns.rs
+++ b/src/adapters/fns.rs
@@ -34,6 +34,7 @@ where
     inner: R,
     next_read: Vec<u8>,
     replacer: Box<dyn FnMut(u8) -> Vec<u8>>,
+    haystacker: Box<dyn Fn(&[u8]) -> Option<usize>>,
 }
 
 impl<R> ByteReplacer<R>
@@ -77,7 +78,7 @@ where
 
         match read {
             Ok(u) => {
-                match memchr::memchr2(b'\n', b'\x0c', &buf[0..u]) {
+                match (self.haystacker)(&buf[0..u]) {
                     Some(i) => {
                         let data = (self.replacer)(buf[i]);
 
@@ -98,6 +99,7 @@ pub fn postprocB(_line_prefix: &str, inp: impl Read) -> Result<impl Read> {
     Ok(ByteReplacer {
         inner: inp,
         next_read: Vec::new(),
+        haystacker: Box::new(|buf| memchr::memchr2(b'\n', b'\x0c', buf)),
         replacer: Box::new(move |b| match b {
             b'\n' => format!("\nPage {}:", page_count).into_bytes(),
             b'\x0c' => {
diff --git a/src/adapters/spawning.rs b/src/adapters/spawning.rs
index 0281963..618a6d2 100644
--- a/src/adapters/spawning.rs
+++ b/src/adapters/spawning.rs
@@ -2,6 +2,7 @@ use super::*;
 use anyhow::*;
 use encoding_rs_io::DecodeReaderBytesBuilder;
 use log::*;
+use regex::Regex;
 use std::io::prelude::*;
 use std::io::BufReader;
 use std::process::Command;
@@ -55,7 +56,7 @@ pub fn postproc_line_prefix(
 }
 pub trait SpawningFileAdapterTrait: GetMetadata {
     fn get_exe(&self) -> &str;
-    fn command(&self, filepath_hint: &Path, command: Command) -> Command;
+    fn command(&self, filepath_hint: &Path, command: Command) -> Result<Command>;
 
     /*fn postproc(&self, line_prefix: &str, inp: &mut dyn Read, oup: &mut dyn Write) -> Result<()> {
         postproc_line_prefix(line_prefix, inp, oup)
@@ -146,7 +147,10 @@ impl FileAdapter for SpawningFileAdapter {
         } = ai;
 
         let cmd = Command::new(self.inner.get_exe());
-        let cmd = self.inner.command(&filepath_hint, cmd);
+        let cmd = self
+            .inner
+            .command(&filepath_hint, cmd)
+            .with_context(|| format!("Could not set cmd arguments for {}", self.inner.get_exe()))?;
         debug!("executing {:?}", cmd);
         pipe_output(&line_prefix, cmd, &mut inp, self.inner.get_exe(), "")
     }
diff --git a/src/adapters/writing.rs b/src/adapters/writing.rs
index d0c95da..a9e9ba1 100644
--- a/src/adapters/writing.rs
+++ b/src/adapters/writing.rs
@@ -2,6 +2,7 @@ use super::{FileAdapter, GetMetadata, ReadBox};
 use anyhow::Result;
 use std::io::Write;
 
+// this trait / struct split is necessary because of "conflicting trait implementation" otherwise with SpawningFileAdapter
 #[dyn_clonable::clonable]
 pub trait WritingFileAdapterTrait: GetMetadata + Send + Clone {
     fn adapt_write(
diff --git a/src/bin/rga-preproc.rs b/src/bin/rga-preproc.rs
index 53a4ace..4638da1 100644
--- a/src/bin/rga-preproc.rs
+++ b/src/bin/rga-preproc.rs
@@ -16,7 +16,7 @@ fn main() -> anyhow::Result<()> {
         std::env::current_dir()?.join(&filepath)
     };
 
-    let i = File::open(&path)?;
+    let i = File::open(&path).context("Specified input file not found")?;
     let mut o = std::io::stdout();
     let cache = if args.no_cache {
         None
@@ -31,7 +31,7 @@ fn main() -> anyhow::Result<()> {
         archive_recursion_depth: 0,
         config: PreprocConfig { cache, args },
     };
-    let mut oup = rga_preproc(ai)?;
+    let mut oup = rga_preproc(ai).context("during preprocessing")?;
     std::io::copy(&mut oup, &mut o).context("copying adapter output to stdout")?;
     Ok(())
 }