How to handle files dropped into a KMP/KMM Compose Desktop project. Tested on Win/macOS/Linux.

FileDropModifierExtensions.kt

/*
 * BSD 3-Clause License
 * 
 * Copyright (c) 2023, Blake Barrett
 * All rights reserved.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 * 
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 * 
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 
 * 3. Neither the name of [Your Organization] nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */
// Originally from: https://github.com/BlakeBarrett/Lore/blob/KMP-Project/composeApp/src/desktopMain/kotlin/com/blakebarrett/extensions/FileDrop.desktop.kt

package com.blakebarrett.extensions

import androidx.compose.runtime.Composable
import androidx.compose.ui.DragData
import androidx.compose.ui.ExperimentalComposeUiApi
import androidx.compose.ui.Modifier
import androidx.compose.ui.onExternalDrag
import java.io.File

interface DropDataResult {
    val droppedData: String
    val file: File? get() = File(droppedData.replaceFirst("file:", "")).takeIf { it.exists() }
}

@OptIn(ExperimentalComposeUiApi::class)
@Composable
fun Modifier.onFileDrop(enabled: Boolean, onDrop: (result: List<DropDataResult>) -> Unit): Modifier {
    return this.onExternalDrag(
        enabled = enabled
    ) { externalDragValue ->
        val result = ArrayList<DropDataResult>()
        (externalDragValue.dragData as? DragData.FilesList).also { filesList ->
            filesList?.let { list ->
                result.addAll( list.readFiles().map { object : DropDataResult {
                    override val droppedData: String
                        get() = it
                }})
            }
        }
        onDrop(result)
    }
}

An example of this in practice can be found here: main.kt#L74