You send twelve photos from an iPhone. They arrive on a Windows laptop as .heic files, and Windows Photos shrugs.
You export a podcast interview as WAV, attach it to an email, and the message quietly turns into a Google Drive link. You download a film to watch on the TV in the living room, and the TV pretends the file doesn’t exist.
None of these files are damaged. They are simply written in a dialect the receiving device never learned, and the fix is almost always the same boring operation: change the container, keep the content.
This is a guide to the format mismatches that actually show up in ordinary use, what causes each one, and which conversion resolves it without quietly degrading the file.
Why mismatches got worse after 2017?
For about two decades, consumer files converged. JPEG for photos, MP3 for music, PDF for documents. Then compression got better, and better compression meant new formats.
Apple switched the iPhone camera default to HEIC with iOS 11 in September 2017, using HEVC compression inside a HEIF container.
The Library of Congress format registry documents the same lineage: a still image compressed with a video codec, stored in an ISO container.
The payoff is real, roughly half the file size of an equivalent JPEG. The cost is that HEVC carries patent licensing, which is why Safari renders HEIC and Chrome and Firefox still don’t.
Three other quiet changes pile on:
- Saving an image from a website often produces .webp rather than .jpg, and plenty of upload forms, print shops and older editors reject it.
- Video downloads increasingly arrive as MKV, a container many televisions and Office-era players won’t open even when the video stream inside is ordinary H.264.
- Email size ceilings haven’t moved. Google’s help documentation still puts the Gmail attachment limit at 25 MB for personal accounts, which a three-minute WAV file blows past without trying.
Format problems are rarely about quality. They are about who can open the file, and the answer changes depending on the device, browser and app at the other end.
A diagnostic table for broken files
| What you see | Usual cause | Conversion that fixes it |
| Recipient says the photos “won’t open” | iPhone saving in High Efficiency mode | HEIC to JPG |
| Upload form rejects an image you saved from the web | The file is WEBP, not JPEG | WEBP to JPG or PNG |
| TV or older player ignores a video file | MKV container | MKV to MP4 |
| Email attachment gets converted to a cloud link | Uncompressed WAV or AIFF audio | WAV to MP3 |
| You can’t select or search the text in a PDF | It’s a scan, so the page is an image | OCR to DOCX or searchable PDF |
| Colleague can’t open a document from a Mac | Pages, Numbers or Keynote file | Pages to DOCX, Numbers to XLSX |
| Logo looks fuzzy when the printer enlarges it | You sent a raster PNG, not the vector | Ask for the original SVG or AI file |
That last row is the one exception worth memorising. Converting a vector to a raster works. Going back the other way does not recover what was thrown away.
What happens when you convert in a browser?
Online converters all follow the same three steps: your file goes up to a server, a conversion engine written for that format pair does the work, and a download link comes back. The differences between services are in format coverage, size ceilings and what happens to the file afterwards.
Convertio will convert a batch of iPhone HEIC photos into JPG on its own servers, so the processing happens in a browser tab rather than on your machine.
That distinction matters more than it sounds: an old laptop that would take twenty minutes to re-encode a video locally does no encoding work at all, because it only uploads and downloads.
The service advertises more than 25,600 conversion pairs across over 300 formats, covering documents, images, audio, video, archives, presentations, ebooks and fonts, and its homepage counter passed three billion converted files some time ago.
The free tier is worth knowing before you start a large job:
| Limit | Free tier |
| Maximum file size | 1 GB per file |
| Conversion credits | 10 per 24 hours |
| Concurrent conversions | 10 |
| Input files | Deleted immediately after conversion |
| Output files | Deleted after 24 hours, or manually from “My Files” |
Text recognition runs on a separate allowance from ordinary conversions, so scanning a long document draws from a different pool than converting your holiday photos.
There’s also an API with PHP examples and a CLI, which is the sensible route if the same conversion needs to happen a hundred times a month rather than once.
Before you upload anything sensitive
Server-side conversion means your file sits on hardware you don’t control for a period of time. For holiday photos and meeting slides, that’s an acceptable trade. For other things it isn’t.
Don’t upload it if losing it would be a problem: signed contracts, medical records, passport and ID scans, payroll files, anything covered by a client NDA.
Use desktop software for those. HandBrake handles video, VLC does quick container swaps, LibreOffice exports documents, and ffmpeg does nearly everything if you’re comfortable with a terminal.
Convertio publishes its retention policy openly, which is more than some competitors manage: input files are deleted immediately, output files after 24 hours, transfers run over secured channels with HSTS, and output files are bound to the IP address that started the job. Read the policy rather than the marketing badge, and apply the same test to any converter you use.
Conversions that cost you quality
Four rules cover most of the damage people do to their own files.
- Recompressing a lossy file loses more. MP3 to AAC, or JPEG to WEBP, means decoding an already-degraded file and throwing away a second round of detail. One pass is usually invisible. Five passes are not.
- Converting lossy to lossless doesn’t restore anything. A JPEG turned into a PNG is a bigger file containing exactly the same artefacts, now frozen permanently. There’s no quality to recover, only file size to gain.
- Upscaling doesn’t add detail. A 720p video exported at 4K is a 720p video with more pixels describing the same blur.
- Keep the original until you’ve checked the output. Open the converted file, look at the last page, listen to the end of the track. Conversion errors love to hide where nobody checks.
If a video’s streams are already H.264 and AAC, moving MKV to MP4 can sometimes be done by repackaging rather than re-encoding, which loses nothing at all. Look at the settings a converter offers before assuming it’s re-encoding everything from scratch.
Settings worth changing
Most conversions run fine on defaults. These four are worth a click:
- Audio bitrate. 320 kbps MP3 for music you’ll keep, 128 kbps for voice recordings you need to email.
- Video codec. Choose H.264 when compatibility matters; H.265 makes smaller files that older devices can’t play.
- Image DPI. Print work wants 300 DPI, screen work doesn’t benefit from more than 72 to 150.
- OCR language. Set it correctly before running the scan. English OCR on a German document produces confident nonsense.
When a converter is the wrong tool?
Sometimes the file doesn’t need converting at all.
If you own the source file, export from the original application instead. Pages, Numbers and Keynote all export to Microsoft formats directly. Word writes PDFs without help. Photoshop saves web formats from the Export As dialog.
Each of these produces a cleaner result than a round trip through a third party, because nothing has to be reverse-engineered.
If the conversion happens repeatedly, script it. A weekly batch of invoices belongs in an API call or an ffmpeg one-liner, not in ten minutes of drag-and-drop.
And if the problem is that a recipient can’t open anything you send, fix the source. Turning off High Efficiency mode in iPhone camera settings stops the HEIC problem at the point where it starts.
Questions people actually ask
Does converting HEIC to JPG lose quality?
Slightly, because both are lossy and you’re recompressing. At default quality settings the difference is invisible on screen. For archival copies, keep the HEIC original and treat the JPEG as a shareable duplicate.
Why won’t my MKV play on the TV?
Almost always the container rather than the video itself. Repackaging the same streams into MP4 usually fixes it. If it still fails, the audio track is probably DTS, which many TVs won’t decode.
Can I convert a scanned PDF into editable text?
Yes, with OCR, and the result depends on the scan. Clean 300 DPI pages convert well. Photographs of pages taken at an angle in poor light do not.
Is there a file size I shouldn’t try online?
Above a gigabyte or so, local software is faster and less frustrating than uploading twice. Anything over that ceiling is a job for HandBrake or ffmpeg.
The pattern behind all of this is simple enough. Formats exist to serve the device that created the file, not the person receiving it. Knowing which conversion bridges the gap takes about thirty seconds and saves the message that starts with “sorry, it still won’t open”.
