Add file_extension fields to BlobType

[ddl-rliu]

See flyteorg/flyte#7009

Tracking issue

Closes flyteorg/flyte#7024 [BUG] [copilot] File extensions are missing when copilot downloads Blob/FlyteFile inputs

Why are the changes needed?

(Keeping this PR in draft until flyteorg/flyte#7009 is merged)

After flyteorg/flyte#7009 merges, adding the new file_extension field to BlobType, this flytekit PR will enable users to configure the file extension on FlyteFile inputs. This addresses the issue where file extensions are missing when copilot downloads Blob/FlyteFile inputs.

What changes were proposed in this pull request?

Add the file_extension field to BlobType. Add annotation "FileExtension", which is used to annotate a FlyteFile when we want to download the file with a specific extension. For example,

# ContainerTask
def t1(file: Annotated[FlyteFile, FileExtension("csv")]):
    ... # copilot downloads the file to e.g. /inputs/file.csv

versus...

def t1(file: FlyteFile["csv"]):
    ... # copilot downloads the file to e.g. /inputs/file

Under the hood, this sets file_extension in BlobType.

How was this patch tested?

Setup process

Ran a workflow locally to test the changes

# Container task python code
def t1(datasetA: Annotated[FlyteFile[TypeVar('csv')], FileExtension("csv")], datasetB: FlyteFile[TypeVar('csv')]): …

# flyte-copilot logs
2026-04-09T22:15:08.506Z: {"json":{},"level":"info","msg":"Successfully copied [1703936] bytes remote data from [s3://ls-engc-flyte-data//25d4f8d8-7eb4-492b-97c4-ab3ce8bdcf74/ae] to local [/execution-vol/flows/workflow/inputs/datasetA.csv]","ts":"2026-04-09T22:15:08Z"}
2026-04-09T22:15:08.540Z: {"json":{},"level":"info","msg":"Successfully copied [196608] bytes remote data from [s3://ls-engc-flyte-data//858fd656-11ec-41ec-b235-b3b2984f230e/ex] to local [/execution-vol/flows/workflow/inputs/datasetB]","ts":"2026-04-09T22:15:08Z"}

Screenshots

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

Docs link

[ddl-rliu]

Bump this after flyteorg/flyte#7009 merges

[ddl-rliu]

Follows same pattern as BatchSize:

flytekit/flytekit/core/type_engine.py

Line 75 in 71194a4

class BatchSize:

[codecov]

Codecov Report

❌ Patch coverage is 45.71429% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 39.44%. Comparing base (c164f35) to head (3edf6e4).

Files with missing lines	Patch %	Lines
flytekit/core/type_engine.py	37.50%	10 Missing ⚠️
flytekit/types/file/file.py	25.00%	9 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (c164f35) and HEAD (3edf6e4). Click for more details.

HEAD has 24 uploads less than BASE

Flag	BASE (c164f35)	HEAD (3edf6e4)
	25	1

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3406      +/-   ##
==========================================
- Coverage   45.78%   39.44%   -6.34%     
==========================================
  Files         317      216     -101     
  Lines       28359    22873    -5486     
  Branches     3015     3021       +6     
==========================================
- Hits        12983     9022    -3961     
+ Misses      15278    13755    -1523     
+ Partials       98       96       -2     

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