pass through mimetype rather than assuming data uri

src/view/com/composer/Composer.tsx

@@ -886,7 +886,7 @@ let ComposerPost = React.memo(function ComposerPost({
   )
 
   const onPhotoPasted = useCallback(
-    async (uri: string) => {
+    async (uri: string, mimeType?: string) => {
       if (
         uri.startsWith('data:video/') ||
         (isWeb && uri.startsWith('data:image/gif'))
@@ -905,7 +905,8 @@ let ComposerPost = React.memo(function ComposerPost({
           .then(blob => new File([blob], name, {type: mimeType}))
         onSelectVideo(post.id, await getVideoMetadata(file))
       } else {
-        const res = await pasteImage(uri)
+        const res = await pasteImage(uri, mimeType)
+
         onImageAdd([res])
       }
     },

src/view/com/composer/text-input/TextInput.tsx

@@ -130,7 +130,7 @@ export function TextInput({
       const file = files.find(f => isAcceptedImageMimeType(f.type))
 
       if (file) {
-        onPhotoPasted(file.data)
+        onPhotoPasted(file.data, file.type)
       }
     },
     [onPhotoPasted],

src/view/com/composer/text-input/TextInput.types.ts

@@ -26,7 +26,7 @@ export type TextInputProps = {
   hasRightPadding: boolean
   isActive: boolean
   setRichText: (v: RichText) => void
-  onPhotoPasted: (uri: string) => void
+  onPhotoPasted: (uri: string, mimeType?: string) => void
   onPressPublish: (richtext: RichText) => void
   onNewLink: (uri: string) => void
   onError: (err: string) => void