You did not specify a name and therefore jsonify will not send back json at all! It will send back a string. This is because proper json is an attribute name with a value. Should be something like: jsonify(message='error message here. (5) Finally, notice that you are not sending back json even though it looks like that is your intent. When a POST creates a resources (e.g., the file) it should return 201 Created. (4) If you want to follow RESTful conventions, then when you finally do upload the file you should return 201 Created instead of 200 OK. Notice also that I returned a 400 Bad Request and added the key message= to jsonify more about that later. Resp = jsonify(message='No file selected for uploading') You don't need to test for '' because an empty string will always evaluate to None therefore it could be better written as: if not file.filename: Resp = jsonify('No file selected for uploading') (3) This could be more Pythonic: if file.filename = '': All of your 500's should be 400's because they are all responding to bad requests, not problems with the server. You might want to read to familiarize yourself with the various HTTP return codes. If the client sent you a bad request then you should be sending back 400 Bad Request and not 500 Server Error since there was no server error. Resp.status_code = 500 <- THIS SHOULD BE 400 Resp = jsonify('No file part in the request') (2) Sending back a 500 is reserved for a serious unexpected server problem. Therefore it will only be called via POST so there is no reason to check (unlike the sample code you may have based this on which declared methods= in which case you do need to check that it is not a GET). You declared the route as methods=: methods=) (1) The check for POST in upload_file() is not needed. I assume you posted because you want feedback on your code: 😉 OpenShift (requires extensive ssh installation of ruby, python, etc.).Google App Engine (Desktop admin app flask can be configured with this repo).WebFaction (SSH paid-web-host flask must be installed by you possible configuration and path issues possible SFTP).Heroku (SSH flask must be installed by you, slightly-difficult-windows-instructions, upgradeable cloud service).(web-SSH web-instant-MySQL-db web-IDE instant deployment of flask, free easy logs github/single-file-upload support SFTP/SSH requires pay).Official Flask website and documentation.

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Flask file upload example