diff --git a/Tusker/Screens/Compose/ComposeViewController.swift b/Tusker/Screens/Compose/ComposeViewController.swift
index 51751f09..c72f4bf9 100644
--- a/Tusker/Screens/Compose/ComposeViewController.swift
+++ b/Tusker/Screens/Compose/ComposeViewController.swift
@@ -493,10 +493,20 @@ class ComposeViewController: UIViewController {
             options.resizeMode = .none
             options.isNetworkAccessAllowed = true
             PHImageManager.default().requestImageDataAndOrientation(for: asset, options: options) { (data, dataUTI, orientation, info) in
-                guard let data = data, let dataUTI = dataUTI else { fatalError() }
-                
-                let mimeType = UTTypeCopyPreferredTagWithClass(dataUTI as CFString, kUTTagClassMIMEType)!.takeRetainedValue() as String
+                guard var data = data, let dataUTI = dataUTI else { fatalError() }
                 
+                let mimeType: String
+                if dataUTI == "public.heic" {
+                    // neither Mastodon nor Pleroma handles HEIC well, so convert to JPEG
+                    let image = CIImage(data: data)!
+                    let context = CIContext()
+                    let colorSpace = image.colorSpace ?? CGColorSpace(name: CGColorSpace.sRGB)!
+                    data = context.jpegRepresentation(of: image, colorSpace: colorSpace, options: [:])!
+                    mimeType = "image/jpeg"
+                } else {
+                    mimeType = UTTypeCopyPreferredTagWithClass(dataUTI as CFString, kUTTagClassMIMEType)!.takeRetainedValue() as String
+                }
+
                 self.postProgressView.step()
                 
                 let request = MastodonController.client.upload(attachment: FormAttachment(mimeType: mimeType, data: data, fileName: "file"), description: description)