this should do the job

unittests/cfdp/handler/testDestHandler.cpp

@@ -3,6 +3,7 @@
 #include "fsfw/cfdp.h"
 #include "mocks/cfdp/FaultHandlerMock.h"
 #include "mocks/cfdp/UserMock.h"
+
 TEST_CASE("CFDP Dest Handler", "[cfdp]") {
   using namespace cfdp;
   EntityId localId = EntityId(UnsignedByteField<uint16_t>(2));

unittests/mocks/FilesystemMock.cpp

@@ -1,6 +1,31 @@
 #include "FilesystemMock.h"
 
-ReturnValue_t FilesystemMock::writeToFile(FileOpParams params, const uint8_t *data) { return 0; }
+ReturnValue_t FilesystemMock::writeToFile(FileOpParams params, const uint8_t *data) {
+  std::string filename(params.path);
+  auto iter = fileMap.find(filename);
+  if (iter == fileMap.end()) {
+    FileSegmentQueue queue;
+    queue.push(FileWriteInfo(filename, params.offset, data, params.size));
+    FileInfo info;
+    info.fileSegQueue = queue;
+    info.fileRaw.insert(info.fileRaw.end(), data, data + params.size);
+    fileMap.emplace(filename, info);
+  } else {
+    FileInfo& info = iter->second;
+    info.fileSegQueue.push(FileWriteInfo(filename, params.offset, data, params.size));
+    // Easiest case: append data to the end
+    if(params.offset == info.fileRaw.size()) {
+      info.fileRaw.insert(info.fileRaw.end(), data, data + params.size);
+    } else {
+      size_t totalNewLen = params.offset + params.size;
+      if(totalNewLen > info.fileRaw.size()) {
+        info.fileRaw.resize(params.offset + params.size);
+      }
+      std::copy(data, data + params.size, info.fileRaw.begin() + static_cast<unsigned int>(params.offset));
+    }
+  }
+  return HasReturnvaluesIF::RETURN_OK;
+}
 ReturnValue_t FilesystemMock::readFromFile(FileOpParams fileOpInfo, uint8_t **buffer,
                                            size_t &readSize, size_t maxSize) {
   return 0;

unittests/mocks/FilesystemMock.h

@@ -1,11 +1,33 @@
 #ifndef FSFW_MOCKS_FILESYSTEMMOCK_H
 #define FSFW_MOCKS_FILESYSTEMMOCK_H
 
+#include <queue>
+#include <string>
+#include <map>
+#include <utility>
+
 #include "fsfw/filesystem.h"
 
 class FilesystemMock : public HasFileSystemIF {
  public:
-  MessageQueueId_t getCommandQueue() const override;
+  struct FileWriteInfo {
+    FileWriteInfo(std::string filename, size_t offset, const uint8_t* data, size_t len)
+      : filename(std::move(filename)), offset(offset) {
+      this->data.insert(this->data.end(), data, data + len);
+    }
+    std::string filename;
+    size_t offset;
+    std::vector<uint8_t> data;
+  };
+  using FileSegmentQueue = std::queue<FileWriteInfo>;
+
+  struct FileInfo {
+    FileSegmentQueue fileSegQueue;
+    std::vector<uint8_t> fileRaw;
+  };
+
+  std::map<std::string, FileInfo> fileMap;
+
   ReturnValue_t writeToFile(FileOpParams params, const uint8_t *data) override;
   ReturnValue_t readFromFile(FileOpParams fileOpInfo, uint8_t **buffer, size_t &readSize,
                              size_t maxSize) override;